In this fifth article I’d like to finish up our discussion of Shooting Modes. (See my fourth article on this site for Part I of shooting modes.) To refresh from Part I we ended with:

Finally what action should we take in terms of our final variable-that of ISO? We will continue this topic in Part II of this article where we’ll talk about Auto-ISO and Exposure Compensation to finish off our discussion of shooting modes. But in the meantime to get started with Aperture priority mode I would suggest one of two ISO settings. If you are shooting with plenty of available light you can probably leave ISO set to base which is most likely 100 on your camera. If available light is poor or you are moving between well-lit and badly lit conditions you will have better results using Auto ISO (letting the camera select the ISO number). Either of these ISO options frees you from having to consider this third variable much while you are shooting right now.

Aperture Priority + Auto ISO

So let’s talk briefly about using Aperture Priority with Auto-ISO and how this function works. Again, in Aperture Priority the camera requires you to choose your aperture and then creates correct exposure by adjusting your shutter speed automatically. If you have your camera set to base ISO-say 100 for example-the camera will only adjust shutter speed-moving it up or down based on correct exposure. With plenty of available light, base ISO should allow for adequate shutter speeds. However, in poor light or when moving between variable lighting conditions we have established that Auto-ISO can be your best option. In this second scenario the camera will first change shutter speed up or down to achieve correct exposure with the aperture you have selected. But the camera assumes you are hand holding your camera and has a built in “safety” feature to prevent image blur due to camera shake. It understands what shutter speeds are too slow for hand held shooting with the lens you are using. Therefore, as soon as shutter speed would have to drop below 1/(1.5xFocal Length of your lens) (for example slower than 1/600 for 400mm lens), it will bump up ISO instead. In other words the camera tries to maintain the old rule of thumb to correct motion blur from camera shake; which is *(1/FL)* and then it errs on the side of a bit more shutter speed. It unfortunately does not take Image Stabilization of lenses into consideration. (Some cameras will use the formula of 1/(2xFocal Length of your lens.) So to repeat, in aperture priority with auto-ISO the camera will adjust shutter speed first, until it drops below the threshold allowed for hand holding your lens, and then it will adjust ISO.

Auto-ISO allows for the configuration of three variables that help you control its parameters. Firstly, you can set a minimum shutter speed-the shutter speed which you do not want your camera to go below because you are attempting to freeze the motion of your subject. This is different from the rule above which is only attempting to prevent blur from camera shake. If I am shooting birds in motion I like to set my minimum shutter speed to 1/1000 (with my largest aperture suitable to the size of the bird to keep it in focus from front to back). Using Auto-ISO the camera is then free to decrease shutter speed in poor light until it hits this minimum threshold of 1/1000, whereupon it will start increasing my ISO automatically. Keep in mind that if you are shooting with a tripod then you may be able to tolerate slower minimum shutter speeds in that you don’t have to compensate for camera movement as well as the movement of the bird. (See Article 3 (Oct issue) for a discussion of camera supports.) If however you set your minimum shutter speed as AUTO (rather than a specific number like 1/1000) then the camera will default to the focal length rule above, which is not usually helpful if you need faster shutter speeds to capture the movement of your subject.

The second configurable quantity in Auto ISO is that of Maximum ISO. This is where you tell your camera the highest ISO you are willing to tolerate. This number depends on the capabilities of your camera and also your own subjective tolerance for higher image noise. Remember, as we increase ISO we increase the graininess or digital noise in the photo. Your tolerance may also vary depending on the type of shot you are attempting; a fine art shot that you intend to enlarge and print can tolerate less noise than a record shot of a rare bird where you simply need to be able to identify the species. I feel comfortable setting the maximum ISO of my Canon 7d MkII at 3200 for most situations. The final variable to configure for Auto-ISO is the lowest ISO you want the camera to choose. This should always be set to the lowest native ISO your camera is capable of-usually 100.

In spite of all the help we are getting from our camera using Aperture priority plus Auto ISO we still need to pay careful attention to the exposure of our images. In most cases, if the camera reaches the maximum ISO you have set and still cannot achieve a correct exposure, it sacrifices the minimum shutter speed. In other words, the camera will not change your aperture (that is the definition of Aperture Priority), and it will also never go above the maximum ISO you set in the configuration. Therefore, it must use a slower shutter speed to expose the image properly. If this occurs and you cannot stop the motion of your subject because your shutter speed is now too low, you have to make a choice: either accept motion blur or raise the maximum ISO you allow or if possible you must open your aperture further.

In addition to motion blur as noted above, it is also possible to get an overexposed image using this configuration. Say for example you are using a low f-stop like f/4 to achieve a very shallow DOF and nice bokeh -if the camera is already using the lowest ISO possible, and a faster shutter speed than is mechanically possible on your camera is needed, then your image will be overexposed. The only way around this is to choose a larger f-stop like f/11 or f/16. So no matter what shooting mode we use we must monitor the camera’s performance and the exposure of our images. There is no one mode that works perfectly in all circumstances.

Exposure Compensation

Finally we should briefly discuss Exposure Compensation which allows us to quickly override exposure settings that were picked by the camera’s light meter. When talking about Auto ISO above we are assuming that we accept the camera’s rendition of correct exposure. Correct exposure means your combination of settings between aperture, shutter speed and ISO have produced a perfectly exposed image; highlights are not blown out and nothing is lost in shadow in an image. But if we check our images and find that the camera has chosen settings that make the image appear to us as either too bright or too dark then we can quickly override these settings to compensate. Camera meters work by evaluating light reflected off subjects and attempting to render the image as a middle gray value. So if the camera is pointed at something very dark, the meter will work the opposite way by brightening up the exposure to a middle gray; conversely it will attempt to darken a bright subject to middle gray. While this works well in most cases, in more difficult lighting conditions like backlighting of our subject, the camera meter might be adjusting the exposure too aggressively. Using Exposure compensation allows us to correct the situations in which the camera’s meter has not in fact rendered the right exposure for an image. In Aperture Priority with base ISO the camera will adjust shutter speed up and down-exposure compensation does not alter ISO or Aperture.

When using Exposure compensation with Auto ISO the camera will attempt to maintain your minimum shutter speed as set above. To provide more light it will increase ISO until it hits the maximum ISO you set-it will then override your minimum shutter speed and start to decrease it. Again, in Aperture priority it will never override your chosen aperture. Below you can see some examples of images with increasing ISO numbers. As we progress from ISO 125 up through ISO 6400 we can see by the crop of each image that the digital noise is also increasing. At ISO 125 the image is crisp and clear with no noticeable noise. Even at 1000 the noise is minimal-the first shot at 1000 is a crop of the original photo and the second shot at 1000 is an extreme close-up of the actual image. By 3200 and 6400 you can see that neither of these images would be suitable for enlarged prints.

I encourage you to give Aperture Priority plus Auto-ISO a try. I think you’ll find that it simplifies your shooting without sacrificing artistic control. See you out there!


Purple Martin Juvenile f/6.3, 1/500, ISO 125

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Prothonitary Warbler f/5.6, 1/1000, ISO 1000

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Prothonitary Warbler f/5.6, 1/1000, ISO 3200

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Wild Coyote f/5.6, 1/640, ISO 6400

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© Copyright 2019 Demayne Murphy


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