Photography / Tips and Solutions

Understanding ISO

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As we noted in our previous segments, Understanding Aperture and Understanding Shutter Speed, photography is the capture of light on a photosensitive surface, be it film or a digital sensor. In order to expose a photograph, we have to control the number of photons that connect with the image surface, as well as control the sensitivity of that surface to the light.

We compared aperture to the eye's iris that opens and constricts the diameter of its opening to limit the amount of light allowed into the eye. Shutter speed is akin to the duration of time your eye is open to admit the light. ISO is similar to the sensitivity of the rods and cones at the rear of the human eye.

In Part Three of this three-part series, we will discuss ISO and then talk about the symbiotic relationship between ISO, shutter speed, and aperture.

ISO

ISO, which stands for International Organization of Standards, is a measure of the sensitivity of film or a digital sensor to light.

If you have ever photographed with film, you are familiar with the different numbers associated with a package of film that you used to buy at the store. Kodak Gold 200, Fuji Velvia 50, Agfa APX 400, etc. The numeral in the film's nomenclature referred to its speed, sometimes given as an ASA number, and the higher the number, the more sensitive the film was to light. This sensitivity was a function of the size of the photosensitive grains on the piece of film, as well as the characteristics of the different chemicals used to create the film.

There are different international standards applied to color negative film, black-and-white negative film, and color slide film, but the numbering system remains constant.

In order to adjust ISO, film shooters have only the option of changing to a different speed film. Once loaded, the ISO is a constant for a given roll of film.

When digital photography entered the fray, the ISO developed speed standards for digital sensors. Thankfully, they assigned the same number system to the digital sensors as they did to the film standards. If you never shot film, you probably are not concerned that the ISO is spoken in the same language, but for those transitioning from film to digital, it is a welcome convenience.

One huge game-changer of digital photography is that, when it comes to ISO, you can change the sensor's sensitivity while you are shooting. ISO is now an electronic function of the camera and not a fixed value as it was with a particular roll of film. So, with the turn of a dial or flick of a switch, you can change your camera sensor's ISO for each image, if you want.

The technical ins-and-outs of how ISO is calculated are fairly complicated and not especially useful to most photographers, but, what is important is how the ISO numbers relate to exposure and exposure values. Just like with aperture and shutter speed, we want ISO to work in EVs, because we are still controlling exposure and we want all three variables to speak the same language.

ISO, like shutter speed, is linear. (Thank the folk at the ISO for saving us from the Inverse Square Law!) A film with an ISO of 200 is half as sensitive to light as the same film with an ISO rating of 400. Double the ISO, double the sensitivity. Half the ISO, half the sensitivity. Easy peasy!

So, assuming all else is constant, if you change your camera's ISO from 400 to 200, or change to a roll of 200 speed film from a roll of 400 speed film, you will introduce a -1 EV shift, as you have just made the sensor, or film, half as sensitive to the given light. ISO 800 changed to ISO 1600; double the sensitivity; +1 EV.

Now let us talk about the "side effects" of ISO. With film, the higher ISO films had larger grains that were impacted by greater numbers of photons. The larger the grains, the "grainier" the image would be. Depending on the type of photo and film, the grain of the film could be a real benefit to the texture and feel of an image. It was something that you had to deal with when shooting film and you could definitely make the grain work in your artistic favor.

Two images of the New York City skyline with the same exposure value. The first image is at ISO 200. The second image is at ISO 6400.

With digital sensors, when we adjust ISO, we are not changing the size of the pixel, but we are increasing its sensitivity electronically by, in layman's terms, increasing the voltage to the sensor. The unfortunate side effect of this is called noise. The higher the ISO, the more digital noise is introduced into your image. Camera manufacturers, while seemingly always adding megapixels to their new cameras, are constantly trying to reduce the amount of noise at a given ISO. While grain could improve a photograph's feel and texture, no one has really proven to me that sensor noise is an artistic improvement in a digital image!

In the digital realm, there are three types of ISO: native, amplified, and simulated. Native ISO is the ISO setting that does not require the camera to increase the voltage to the sensor. The camera's native ISO is not usually published by the manufacturer in the specifications for a given camera, but a quick Internet search will show you what your particular camera's native ISO is. Amplified ISO is an ISO that requires an increase in voltage to the sensor to achieve. This is when noise starts to creep into your images: the higher the ISO, the greater the noise. And, finally, simulated ISO is when the camera uses a software algorithm to simulate even higher ISOs. Regardless of whether the increased ISO is amplified or simulated, you will see an increase in noise over the native ISO setting. And, regardless of the type of ISO, the numbers still adhere to the same linear relationship with EVs.

It's all EVs!

So, let us bring this exposure journey full circle, shall we?

In the three segments we discussed how, in order to control the amount of light striking a piece of film or digital sensor, we have two physical controls that we can utilize inside the camera—aperture and shutter speed. We can further control the sensitivity of the sensor by adjusting the ISO on a digital camera (or by getting a roll of film of a different ISO).

We also discussed how each of these three adjustments can be quantified into EVs and that by doubling or halving the amount of light (aperture and shutter speed) and by doubling or halving the sensitivity of the photosensitive surface (ISO) we can adjust EVs up or down.

In order to maintain the exposure you are searching for, be it a proper exposure or intentional over- or under-exposing of an image, when you allow more photons into the camera, or increase the sensitivity of the film or sensor, you will need to compensate in the opposite direction with the other camera controls.

So, let's make this all practical by looking at a few photographic scenarios where you would adjust one of these three elements.

A wide aperture blurs the New York City skyline in this portrait.

1) Outdoor portrait. I am outdoors in sunlight and I want to take a photo of a friend. I saw some photos the other day that featured a lovely blurred background behind the model and I want to emulate this characteristic of those photos. In order to do this, I know from reading B&H Explora that I need to open my aperture as wide as I can to shorten the depth of field. When I turned the camera on in Auto mode, the camera said it was going to take a photo at f/8 and 1/500th of a second. I want to shoot at f/2.8 to get my blurred background. I am outside in sunlight, so my camera should be set at its native ISO and there should be no reason to adjust that. I have a few options now:

  • I can switch my camera to Aperture Priority mode and let the computer determine the correct shutter speed for a proper exposure.
  • I can switch to manual mode and, in my head, figure out that when I went from f/8 to f/2.8 I made a +3 EV shift to my exposure as I have opened up the aperture by 3 stops (f/8 to f/5.6 to f/4 to f/2.8) and thereby allowed more light into the camera. To maintain the balanced exposure using shutter speed, I need to reduce the amount of time that the shutter is open by 3 stops in the other direction. Therefore, I will change my shutter speed from 1/500th to 1/4000th of a second (1/500th to 1/1000th to 1/2000th to 1/4000th).

Mount Shuksan in North Cascades National Park, taken from a US Navy Sikorsky UH-3H Sea King helicopter. A narrow aperture allows greater depth of field to keep the helicopter tail and mountain relatively sharp.

2) Landscape Image. I am taking a picture of a city skyline or beautiful snow-capped mountain range and my friends want to be in the image in the foreground and are forgoing the opportunity for yet another selfie (thank goodness!). I have my SLR camera and we can get a better shot than with our phones. I want to keep my friends sharply in focus but also show the cityscape/mountain in focus, as well. The camera is suggesting a photo at f/4 and 1/1000th of a second at the native ISO. I want to increase the depth of field to keep my foreground friends in focus while making sure the background does not become a bunch of unrecognizable blurry shapes—f/16 might work. My options:

  • I can use Aperture Priority mode, change the setting to f/16 and let the camera determine the best shutter speed.
  • I can, using Manual mode, shift to f/16. This will reduce the size of the aperture opening by 4 stops that I must now compensate for with shutter speed. Why can't I compensate with ISO? Well, in theory, you can, but if your camera is already on the native ISO setting, you probably will not be able to desensitize the sensor or film by the needed 4 stops in this scenario. So, you will have to compensate with a slower shutter speed. Four stops from 1/1000th is 1/60th of a second (1/1000th to 1/500th to 1/250th to 1/125th to 1/60th).

High shutter speed freezes the action on a football pitch.

3) Kids' Soccer Game. You need action shots of your kid's heroics on the local youth soccer pitch and want to try out your new telephoto lens. The key to freezing action is a fast shutter speed. Because the light on the field might be very dynamic, this is a good opportunity to use either Shutter Priority mode or Aperture Priority mode and let the camera do the EV math for you at high speed. This may be a scenario where an ISO adjustment will come into the fray.

  • If you use Shutter Priority mode, you can set you shutter speed to something very quick, say 1/1000th of a second. You will see the camera compensate by opening up the aperture near or all the way to its maximum width to let in as much light as possible for the quick shutter opening.
  • If you use Aperture Priority mode, you can open up your aperture as much as possible. The camera will compensate by giving you the fastest shutter speed available for that amount of light.
  • So, hold the phone. It is a cloudy day and you noticed that in Shutter Priority mode your images (if you are shooting digital and can instantly review the photos) are looking dark even when the computer is giving you your maximum aperture. In Aperture Priority mode, your images are looking blurry because the shutter speeds are not sufficient to freeze the action. What do you do? This is where you can increase the EV value for the given aperture or shutter speed by making the sensor more sensitive to the photons. So, dial up the ISO number by one or two or three or more EVs and see if that helps brighten your Shutter Priority photos or sharpen your Aperture Priority pictures. Be careful, turning the ISO up too high will result in unattractive noise!

 A low-light, handheld image from a New York street. After opening my aperture to f/1.8, I had to increase my ISO to 800 to get the shutter speed sufficiently quick to avoid camera blur in the low light.

4) Handheld Low-Light Photo. One of the things that many photographers dream of is the ability to capture a photograph in a dark space, such as a bar or restaurant, without using a flash for extra light or a cumbersome tripod. In the past, you needed a very fast (high ISO) film and a lens that allowed a very large aperture. The lens portion of the equation remains true in the digital age, but not quite as much as the low-light performance of high-ISO digital images is getting better and better. So, here is how I approach the image:

  • Depending on how dark the environment is, how large I can make my aperture, and how long I have to set up the image, I will either start at the native ISO or bump it up one or two EVs. I will then open my aperture to its maximum diameter and see what kind of shutter speeds the camera is giving me in Aperture Priority mode. If they are still too slow for me to eliminate the blur caused by camera shake, I will turn the ISO up even more.

You may have noticed that in all of the above examples I was adjusting the camera in full-stop or EV increments. Many cameras let you select and change exposure settings (ISO, Aperture, Shutter Speed, and Exposure Compensation or Shift) in 1/2 stop or even 1/3 stop increments. If your math skills are on par with mine, I recommend setting your camera to work in full or 1/2-stop increments to simplify the calculations. I'm sure there are those who will argue against that, as you might lose some of the precision-exposure capabilities offered by 1/3 step adjustments, but I accept tradeoff in the name of easier math.

Conclusion

I hope you enjoyed this series on exposure and how to control exposure in a camera. One key to getting the photographs you want is knowing how the different variables not only affect each other when exposing an image, but also knowing the different side effects of adjusting each variable (depth of field, motion blur, noise, etc). As I wrote at the beginning, photography is art. You should always feel free to experiment with changing your camera settings in order to achieve the objectives of your artistic vision by altering exposure, depth of field, motion blur, etc. Good luck, have fun, take great photos, and make art!

70 Comments

Really helpful and so clearly explained. Thank you!

You are very welcome, Patrick! Thanks for reading!

So my A7SII's Sensor's Native ISO is 3200, but I can't take shots at day very well because of that, how do I do longer exposusures?

Hey Stanley,

I was under the impression that the A7S II's native ISO was ISO 1600. A true low-light machine, I guess!

Well, if you stop your lens down and you are still getting through-the-roof shutter speeds (or not getting the DOF you want) you''ll just have to dial down the ISO to a more manageable setting.

The disadvantage of the artificially low ISO settings is that you lose some dynamic range (but probably not much). You will not see increased noise at lower ISOs like you do at boosted ISO.

First world problems, right, Stanley? 

Todd
great series . TYVM

I am trying to take portraits in sunlight w fill flash (to get under the hat and show the eyes) but either the camera will say I dont need the flash or I can force it to flash w some of the scene modes and try to physically lower the flash but it amlost always overexposes and washes skin tones out-or under and red...what am I doing wrong? ty

Hi Billy,

What kind of camera are you using?

If the camera allows it, you should be able to shoot on full manual mode and fire the flash on command. I would take a shot and then check the LCD to adjust your exposure with the flash firing.

Thanks for stopping by!

Why does my Cannon D40 Only go up to ISO 1600? How can I u8se it for Concert Photography when a flash is forbidden? thanks 

Hey Jeff,

Older digital cameras only had a portion of the ISO range available on today's cameras. Why only ISO1600? That is for the Canon engineers to answer.

For low light scenarios, when shooting handheld, the only way you can get more light to the sensor is to shoot a lens with a large maximum aperture. If you are shooting zoom lenses, look for at least an aperture of f/2.8. For prime lenses, try lenses that open up to f/1.2, f/1.4, or f/1.8. You can get f/1.8 prime lenses at very reasonable prices—no zoom, but they let a lot of light in!

Let me know if you have follow-up questions! Thanks for stopping by!

Todd thanks; I have a cannon 28 x 135mm Lens; Tameron 70 x 300mm Lens and my largest is HQ Optics 500mm Mirror Lens  ! thanks Jeff 

Hey Jeff,

Yeah, you are definitely fighting physics with those lenses. They aren't bad lenses at all, but none are going to maximize your light-gathering potential. The best thing to do is to get something like a 50mm f/1.8 or 85mm f/1.8 lens and get much much closer to the stage with the lens aperture wide open and start shooting! 

With the mirror lens and the Tamron, you are in the telephoto to super-telephoto range...lots of issues with camera shake with shutter speeds slowed by the dim lights of the concert hall(s). So, getting a shorter focal length prime lens will help with that as well.

Good luck!

Hi, sometimes I see a 503 site message when I browse your webpage. I thought you may wish to know, best wishes

Thanks, again, Murv!

Very good series about how to control exposure, Thank you.

Thank you, Darek! Thanks for reading!

Todd,

Thanks for writing such a great series on camera exposure! I've been teaching an Introduction to Video seminar, and your series has been more helpful - in terms of providing a clear description without oversimplifying - than most of my video books. You should write your own book on this subject!

Hi Shimon!

Thank you very much for the compliments! I very much appreciate it.

Should I sign you up for an autographed copy of the book? :)

Thanks for reading!

Great series and educational for Old men with Cameras as well as keen starters in this amazing craft of photography. We can get to obsessed with the technical that we forget about the creativity and the enjoyment of capturing an everlasting moment. With Social media and so many pictires taken today, the lifspan of 90% plus of all images is less than 5 seconds. For those who do ir as a serious hobby or as a means of income and a full time craft, the key is to understand our equipment and optimise. The search by the manufacturers continue for optimisation of mega pixels, ISO levels, focus speed, 4K video and what comes next. Some of these aspects are needed for the advancement of the commercial work, however the best work still is produced by those that know how to optimise that tools. Moving from film to digital, the ISO optimisation is the biggest challange, each camera has their limits on ISO performance and so has every photographer ito how far you want to push it. Thanks for a great series

Hey Jakes,

Thank you for the awesome comment! It sounds like we are on the same page!

There is a time and place where low-noise/high-ISO is good for my photography (and that of many others), but 98% of my images are taken at ISO 200...so the technology is usually lost on me. For some, it is critical to their work as it certainly expands the possibilities of natural light shooting in unfavorable lighting environments.

Regardless, amazing high-ISO performance does not a great photograph always make.

Thanks for stopping by!

'One huge game-changer of digital photography is that, when it comes to ISO, you can change the sensor's sensitivity while you are shooting.' - Not really. It depends on what you mean by 'sensitivity', but to a first approximation the sensitivity of a digital camera does not change (with a few exceptions which use the DRPix technique to make a step change somewhere in the ISO scale). ISO is 'output sensitivity', it has nothing to do with the sensor, it just determines the grey scale value that will be used for a given exposure measured by the sensor.

Hi Bob,

I believe you are correct. However, "sensitivity" seems to have been accepted as the best way to describe the function of ISO as part of the exposure triangle. In fact, every major camera brand uses the term when describing the concept of digital ISO.

I had a long back-and-forth over email (see below) with a gent about how it is really "amplitude" and not "sensitivity."

I find that when I teach photography to beginners, there is absolutely zero to be gained by getting into the weeds about ISO. Many find the concept difficult enough without delving into wiring diagrams and electronics!

However, I appreciate you taking the time to read and leave a comment! Knowledge is good! Thanks!

Iso does not stand for the International Standards Organization. It is the International Organization of Standards. IOS in America/UK, OIN in france. Go to the International Organization of Standards web site Iso.org. Go to the tab "About us". Then roll down to " It's all in the name".

From iso.org:

"It's all in the name

Because 'International Organization for Standardization' would have different acronyms in different languages (IOS in English, OIN in French for Organisation internationale de normalisation), our founders decided to give it the short form ISO. ISO is derived from the Greek isos, meaning equal. Whatever the country, whatever the language, we are always ISO."

Hey Dan,

Thank you for the sharp eye and the correction! We will change the text to reflect the correct name!

...just when I thought I was perfect! Darn!

Thanks for reading!

You might not be "perfect", you just thought you was. We, fellow man, are neither BUT also could think that same thought. Ain't that right?

Regardless, perfection is a worthy goal, Franco!

Thanks for reading!

I had a unfair advanage. I used to have to teach this  and other stuff to employees of companies adopting ISO standards.

Keep up the good work.

Thanks, Dan! We should always take advantage of unfair advantages!

For shooting in low-light conditions where I want a photo even if it may end up not being a good one, I have been using Automatic ISO.  I have found this very helpful when shooting in cities at night or photographing indoor concerts (which I get asked to do more than I want).  In manual mode, I set the aperture at the widest I can tolerate for the depth of field I want, and I set the shutter speed at the longest exposure I think I can hold steady with image stabilization.  Then, with Auto ISO, the camera determines the exposure and sets the ISO to get the shot.  Sometimes the high ISO results a photo that is too grainy, but usually I get good enough results to impress my friends.

Hey Glen,

Good technique! Auto ISO has become a very viable tool in the digital photographer's quiver. If you want to avoid the noise associated with very high ISO, be sure to set the Auto ISO upper limits to something that will not produce an unfavorable amount of digital noise.

Also, keep an eye on it. I have seen, on my cameras, more than one instance of the ISO being abnormally high when I didn't feel that a high ISO was required.

Thanks for writing in, Glen!

At last I understood those terms, ISO, aperture! Such a relief. :-)

Thanks a lot!!!

You are very welcome, Orli! Thank you for reading!

Guru 

After reading many articles , I finally got this mathematics in my head. A human eyes n camera is grt way to explain. Aperture vs zoom in or out ... Have a relation isn't it..? 

Guru in India is ancient which meant teacher , :) 

Thanx Todd

Hi vishal,

Thank you for the question for the Guru! 

Question: Aperture vs. zoom in or out...Is there a relation?

Answer: That depends on the lens.

Some zoom lenses have what are called variable apertures. This means that the maximum aperture is not consistent throughout the zoom range of the lens. When the lens is zoomed to its widest angle, the maximum aperture will be at its widest. As you zoom in towards the telephoto range of the lens, the maximum aperture will get smaller. When shopping for a zoom lens, look for the name of the lens to have two numbers for the aperture. Example: 18-200mm f/3.5-5.6

Zooms with constant-apertures will have the same maximum aperture through the zoom range. The names of these lenses will only have one aperture number. Example: 70-200mm f/2.8

I hope this answers your question. Thanks for reading!

Great articles and aperture video, Todd, et.al. We have some HS summer interns and these materials will provide them with a good introduction to exposure, backed-up nicely with enough of the physics, math, and cross-references to film-based photography.

You make mention of "camera shake" a couple of times, but didn't go further. I know this can be a frustrating occurrence of novice photographers as they remember a sharp image in the viewfinder, but the camera "took a blurry picture." A brief discussion (like you did for the "side effects" of aperture) would go a long way--especially the relationship between focal length and the appearance of camera shake. [Back in the dark ages (before VR), we had the shutter speed rule-of-thumb of 1/focal length for hand-held shooting.]

Hello SciTech,

Thanks for the compliments and thanks for reading!

You are correct, I guess I could have expanded on camera shake a bit more. I will pitch an article dedicated to the subject to see if we can give it a good round turn.

I use a Canon XSI camera.  How do I find the Native setting?  I guess I don't understand the term.

Hi doug,

The "native" ISO setting is the one where the camera does not have to boost (or reduce) the ISO signal electronically. It is kind of like the idle throttle setting on a car - if you want it to run faster or slower, you need to adjust the fuel flow.

The Canon native ISO is usually 100. I did a web search to see if your XSI was different than this, but I could not find anything.

I hope this helps you out. Thanks for reading and asking a good question!

Thank you for that great clear explained tutorial Todd. Very helpful for a newbe like myself.

Hi Rey,

Thanks for reading and thanks for the comment! Have a great weekend!

Hi Todd - Quite informative. I recently moved from a canon powershot (point and shoot) to a Nikon D5300, and am enjoying playing around with the settings and examining the results. Since this is my first foray into DSLR's i wanted to get your inputs on increasing the sharpness and quality of photos for indoor fast action photos. We hosted our daughters birthday recently at a indoor play house and i must admit i totally goofed up the fast action pictures of my daughter and her friends jumping on the trampoline and air castles. This was a relatively low lit environment. I wanted to get a feel of what settings i should have used for such situations (especially close ups and mid range pics). I only have a Nikor 18-140 lense

Hi Raj,

Thanks for your comments and question. Congratulations on the new camera too!

First of all, do not be disheartened by the lack of success with the indoor action shots. Action photography, indoors, with poor lighting is one of the most impossible missions for a photographer and his gear. To help you out, let us look at challenges, causal factors, and possible solutions.

Your primary issue is going to be having the camera operate at a sufficient shutter speed to freeze the action. My guess (based on limited experience with jumping kids) is that you will want at least 1/500th of a second to really freeze the action. The first step in getting this is to open your aperture to its maximum. If you are in Program or Auto mode, you can do a program shift to open the aperture. If you are in Aperture Priority mode or Manual mode, you may command the aperture to open as well.

With the NIKKOR 18-140mm lens, you have a variable maximum aperture from f/3.5 to f/5.6. This means that at the telephoto end (near 140mm focal length) the lens can open to f/5.6. At the wide angle end of the zoom range, you can get open to f/3.5. This is definitely a factor in how much light is going to get into the camera. To get the fastest possible shutter speeds with that lens, you need to stay in the wide angle region, so stay wide and get closer to the kids or suffer the loss of light from the variable aperture. Also, the further you zoom into the image, the more that your camera movement will be amplified by the increasing focal length - another reason to stay wide.

Once you open your aperture all the way you will likely find that you still are not getting sufficient shutter speed to freeze the action. You have a few options now. First, you can increase the ISO setting and permit the faster shutter speeds. The tradeoff is increased digital noise. The second option is that you can add light to the scene with the camera's built-in flash and/or additional strobes and lights. If you are using the camera's flash or other strobes, you have to pay attention to the camera's sync speed for flashes so that you do not catch the shutter mechanism in your image (a possible topic for another article).

The last solution is a hardware change. I am a huge proponent of owing and using a quality 50mm fixed focal length lens for many reasons - indoor action being just one of them. I won't reiterate it here, but check out my article Going Beyond the Kit Lens to see my soap box about the 50mm lens. A large-aperture 50mm lens will allow you to let a lot more light into the camera than the 18-140mm lens and it might help you reach those shutter speeds with a minimal bump in ISO. The (usually) more expensive hardware option is the purchase of a fixed-aperture telephoto lens, usually f/2.8 or wider, that will also help you get more light into the camera.

Final thoughts for you...Indoor, low-light, action photography is sometimes all but impossible, so know the challenge before you take the challenge. Very few of us can pull it off without specialized gear. If you simply cannot get to the shutter speeds you need to freeze the action while keeping the ISO noise manageable, I would encourage you to either step back and just enjoy the action with your own eyes and remember the moment in your mind or, maybe better yet, embrace the blur and try to get some really creative shots of jumping children streaking across your images. You never know what kind of cool images you might get when you don't fight the physics of the image.

I hope this helps. Thanks for reading! Feel free to ask follow-ups!

A great trio of articles that really set forth a difficult concept in an easy to follow style. Your explanations are great. Your comment about the advance in ease of photography with the development of adjustable ISO in digital cameras is right on. Incidentally, my first SLR was a Kodak Retina Reflex using Kodachrome II.

How about some more great articles!

Chris Preu

Thanks for reading Explora, Chris!

More articles to come!

I knew the basics of ISO, but this article taught me a lot more.

Thanks again again, Kent! I appreciate your words!

liked it much

Thanks, rio!

Hi

For scenario 2 you wrote 'Why can't I compensate with ISO? Well, in theory, you can, but if your camera is already on the native ISO setting, you probably will not be able to desensitize the sensor or film by the needed 4 stops in this scenario.'

Why would this be the case? I would assume increasing the ISO would be the alternative to reducing shutter speed (or a balance of the 2)

Hi Richard,

Your math is correct.

However, if you have your camera set a the native ISO, it is generally at its lowest setting (or maybe one stop above its lowest setting). Therefore, if it is at its lowest setting, you cannot go lower and therefore cannot compensate for the exposure with an impossible-to-get lower ISO.

So, in scenario two, we are closing the aperture four stops. Assuming our ISO is at the native setting (where it should be on a bright sunny day), the only way to compensate for the smaller aperture is to increase the amount of time that the shutter is open.

I hope this answers your question.

Thanks for reading!

thanks for your reply Todd

however, in this scenario wouldn't you want to increase the ISO to increase sensitivity to compensate for the lower EV caused by closing down the aperture? So taking ISO from say 100 up to 500

Great info!  Well done!

Now to  use it.

Mike

Thanks Mike! Get out there and use it!

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