Friday, August 17, 2012

Exposure part II: Know Your Limits

Today I will talk a little about unsharp pictures. Most people are happy with their camera and seem to get sharp, colorful images every time, even with a cheap point-and-shoot or a cell phone camera. At least, until they try to take a picture at a low light situation. This is usually when the blurriness starts.

Of course, buying a better (more expensive) camera might help. It might not (sometimes it's too dark). Maybe, just maybe, you just didn't make the right choice of aperture, shutter speed and ISO.

I don't intend to specify what the "correct exposure" for every situation is. For that we have a built in light meter, and a little screen on the back of the camera.

I do want to show some examples of what kinds of blurriness or noise we get from badly chosen exposure settings. Once you know what they look like, you might be able to get sharper images in the same situation, or at least know when too dark is just too dark.


If you don't know what 'aperture', 'shutter speed' or 'ISO' means, I suggest reading the previous post on this subject here.

For non-DSLR owners I suggest starting to use M mode as much as you can. Usually you can tell the brightness from the screen, and it will become much easier for you to notice when the exposure is too long (for most pocket cameras this is around 1/4 second, handheld).

For DSLR users I really recommend at least going over the pictures once on the computer, using any program that lets you see the settings you used for each picture. It helps to get feedback (and when you really want the sharpness you deserve from a good SLR you should look at the results on a big screen) along with the settings you used. This gives you a feeling of what you can get away with in low light situations and what really is tack sharp.

So what possible kinds of blur could we have?

Bad Focus or Depth of Field problems

If you find your subject to be blurry, but the background (or any other object in the frame) to be a little sharper, then you probably just got the focus wrong.

This happens a lot. You take a bunch of pictures, maybe one of them the focus point was just missing the person and you get slightly blurred portrait and a sharp background. Look for this on the big screen when you get home. It may be subtle, and hard to see unless you zoom in (on the camera screen, without zooming, it is really hard to notice). Once you see it, though, you feel pretty stupid for not focusing right.

Solution: make sure you are in focus. Make sure the focus point is on the person, make sure you didn't switch to manual focus by accident and make sure you don't have something in the foreground distracting the auto focus (in which case that will be in focus instead of your subject).

Taken at f/4.0 - Focus is on the nose and not the eyes
Again at f/4.0 but this time the focus is on the right place. 

Another, related problem is when you get only part of the subject in focus. This happens most often in macro work but also in portraits. The person's face is focused well but his ears are out of focus. Sometimes that's just fine, but if you take a picture of a bug and only his head is focused without the body, you might be missing some details. Once more, this is hard to notice on the camera back screen. Zoom in or check it at home to see if your settings were good...

Going to f/11 we get larger DOF. even though focus is on the nose (see full resolution) it looks ok. 

To get more of your object in focus use a smaller aperture. At f/2.8 only a thin screen, parallel to the sensor, is in focus (this is the focal plane). When you go to f/22 a lot of the volume in front and behind that plane is also in focus. This is the depth of field (DOF) you must have heard about. Large depth of field allows more things to be in focus, and you get it by closing the aperture.

In general, at closed aperture the focus is more forgiving, and even if you miss the point of interest by focusing at something close (example: focus on the nose instead of the eyes of a portrait), at small aperture they will both be in focus. So smaller aperture helps get things in focus, and generally gives sharper images (see next item).

If you do want the background blurred use a large aperture. If you don't have enough light (the shutter speed or ISO is too high) use a large aperture, but know that it comes at the price of your depth of field.

Overall Softness (Aperture / Lens problems)

The best aperture is usually between f/5.6 to f/16. Most lenses will give good results in this range. For pocket cameras the aperture usually has little effect on the image quality, but avoiding the widest aperture is always a good idea.

If your image is overall soft, and if you notice you consistently get sharper results when the aperture is more closed, then you might need a better lens, or just try to work at smaller apertures.

I found, after a couple of years of using it, that my Tamron 17-50mm f/2.8 lens (for Nikon) is noticeably soft at f/2.8. By f/4 it is almost perfect and at f/5.6 it is completely sharp.
Nikkor 50mm f1/.8 and a ruler at an angle. Taken @ f/1.8 shows overall softness and low DOF
Same setup and same lens, at f/8/ Much more DOF but also noticeably sharper. 

For most uses I could get away with it, but I will prefer going to ISO400 on my D90 and clean the noise a bit (see below) than take images at f/2.8. If I have no choice, and I don't need super sharp images, I will take it at f/2.8 and use the images for web upload with no problem.

Remember: when you have lots of light, using a slightly smaller aperture almost always gives sharper images.

NOTE: At very small aperture (around f/18 or f/22) you will get some softness due to diffraction. There's nothing we can do about it: its physics. Don't go over f/16 unless you need lots of DOF. If you still do go there, expect some overall softness. 


1) If you have lots of light, use f/5.6 up to f/11. If you want lots of DOF (to get everything in focus) go to smaller aperture as much as f/22 (but expect some softness starting around f/18). If you want a blurred background (small DOF) then use as large an aperture as you got. Consider using one stop smaller than maximum aperture to avoid lens softness on that end (use f/4 instead of f/2.8 if you feel your lens is less that tack sharp when wide open).

2) If you don't have a lot of light: the aperture is usually what you start with, opening up as much as you dare. In most indoor settings even at f/2.8 you will need to compromise shutter speed or ISO so an aperture wide open is usually what you will have to go to in dim lighting.

Motion Blur (too slow shutter speed)
In full daylight we will rarely have motion blur, maybe only if we have a very long telephoto lens or if we really close the shutter down a lot (for macro maybe).

Shutter speed really depends on the focal length and on whether or not you have an image stabilizer. For non stabilized lenses the thumb rule is 1/(focal length).

Example: I am shooting with a lense of 50mm, I can usually get sharp images at 1/50 second. With a  200mm lens I can probably get good shots with 1/200 or faster.

Most pocket cameras, at the least zoom possible (what focal length is that?!), with an image stabilizer, will allow you as slow as 1/4 seconds, maybe a little more if you have a steady hand.

f/2.8 @ 1/60s, very little motion blur
f/5.6 @ 1/15, blur is visible at full resolution

As you can see, the right image is smudged in a certain direction, and has blur in all areas of the picture. This hints that we have motion blur (rather than noise or bad focus). See the full resolution images to really see the difference. Also, the left image has less DOF.

Notice that motion blur will sneak up on you. Sometimes you don't see it until you look at the computer screen at home. This is why we want to be able to recognize this kind of blur so we know what shutter speed is too slow for our camera/lens/focal length. Once you know what shutter speeds you can get away with you can avoid taking pictures at too slow a speed (pics that look good on the screen but later turn out to have motion blur are very frustrating).

If we know that we are going to have motion blur, how to fix it?

1) Get a tripod, a fence post, a boulder or anything you can use to stabilize the camera.
2) Increase ISO or open the aperture to get shorter exposures (this is what the camera often doesn't do very well, resulting in blurry images when in Auto)
3) Take multiple images in burst mode and hope one of them has less blur (if the blur is slight this method actually does work sometimes)
4) Take the picture deliberately darker (use EV-1 for example), make it creatively shadow dominated, and pretend this was intentional and not a last resort for not having enough light.
5) If all else fails, use a flash*

Notice that motion blur is extremely difficult to get rid of (Photoshop 6 is rumored to be able to handle it, but don't count on this just yet). A small amount will be passable for small print or low resolution web albums, but really you should try to avoid it by making better choices with the camera settings.

A note on stabilizers: Although these things might give you 3 or 4 stops of shutter speed (e.g. from getting away with 1/50 second  to managing  1/4 ~ 1/3.2 second with the stabilizer), there is a limitation on this too: moving subjects, like people or cars, will be blurred regardless of your stabilizer. If you get pictures where the background is steady but the person is blurred, then no stabilizer or tripod will help (unless you can get the person sitting still, which is sometimes pretty hard to do). In this case high ISO or flash are your best options.

Taken with tripod using f/11 @ 1/4 sec.
The background is sharp, the plant has motion blur thanks to a fan blowing. 

This kind of blur (subject motion) happens also when you use a tripod or stabilizer. In addition, you will find general motion blur even when using a tripod. This, of course, is because you are doing it wrong... use a remote or shutter delay and maybe even a better tripod (or at least weigh it down somehow).

Image Noise (high ISO)

If you can't manage a good combination of aperture and shutter speed at a low-light situation, or if you must have a very small aperture (for DOF purposes)  or if you need a super fast shutter speed (because you are using a very long lens, etc.) then consider increasing the ISO.

Most cameras have the lowest (or near lowest) ISO setting as their default. My old Sony had 100 as default and could do 50 as well. My Nikon has 200 as default but can drop to 100 if needed. Try and shoot everything at the sensor's default settings or less because any more than that introduces noise.

Noise shows up as little speckles of light or color in random places all over the image. Usually they are easier to see in darker regions of an image.

ISO200 @ f5.6, low DOF but little noise
ISO3200 @ f/22 has large DOF but massive noise
The noise here is hard to see. Open full resolution. 

This can be mitigated by using noise reduction, either in camera or in post (which is usually better). For web upload or small prints you would probably not even notice ISO raised by one stop (like going from a default 200 to 400). The images are a little grainy when enlarged, but a little noise reduction applied to them will fix that.

Note that using noise reduction smooths out an image but also reduces detail and textures. Sometimes a lot of noise reduction is good to get smooth skin, but expect some loss of detail.

Same ISO3200 as above, with 50 noise reduction
in Lightroom 3. Details are smoothed out considerably. 

My Nikon gives passable results at ISO 800 if I use some noise reduction, or if I don't mind the pictures to be grainy. Sometimes, especially in BW images, you can let the noise look like grainy film and get away with it. When you want clean, sharp, and detailed images, high ISO really messes the picture up.

My camera, at ISO 1600 gives pictures that cannot be repaired back to passable, and I will not shoot this high unless I don't have a choice. I will try and use a slightly longer exposure (trying to hold steady and avoid motion blur). If there's still not enough light I already know the image will not be a good one (and might pass it up or use flash*)

New DSLRs have improved their ability to shoot in high ISO. This is done through better sensors and some built in noise reduction. Though this improves things, it is important to know your camera and know at what ISO the images become unusable.

Make sure you check this for some pictures of not-so-well-lit subjects, as the darker regions show the most noise. Make sure to use whatever noise reduction you can and see if the smoothing of the details is not too obvious.

This is something you need to learn quickly, as the next time you use high ISO in a low light environment, you might come home and find a grainy picture that is beyond repair because the ISO was too high.


If you can use slower shutter speed or larger aperture, do so. If you can't (say if you suffer too much from blurring because of those settings) increase your ISO, but not too much. Make sure you know how much is too much.

If you still can't make the image, use a tripod, turn on a light, or use a flash*.

Nikon D90 with 50mm f/1.8 lens, taken by my cellphone Galaxy S 1. Both noise and motion blur are quite obvious.

Concluding remarks:

If you really must take pictures under less than ideal lighting conditions, or if you just need to go to an extreme in aperture or shutter speed for some purpose, try to remain inside the reasonable limitations of what your camera can do. Don't use just a high ISO or just a very long exposure. Most of the time a little compromise in all three fields will give the best results.

This little chart is something every camera calculates on its own, but you should be the one choosing which option to go with. Say you measure light and get ISO400 with f/5.6 at 1/30s. You can play around a bit. Usually the sweet spot is somewhere in the middle. If you have a wide angle lens 1/30s is fine. Maybe you don't lose sharpness on that lens at f/4 and can get a better ISO. Maybe you have a 50mm on and you need 1/60s to avoid motion blur, but also you must have large depth of field so you go for f/8, which leaves you at the upper right corner at ISO1600 (which is pretty awful). A tripod/stabilizer lets you go all the way down to 1/8s with ISO 200.

The corners of this chart are usually pretty bad, and if you have a really low light setting you won't find any good options anywhere.

The following pictures were taken handheld under indoor lighting, using a D90 and Nikkor 50mm f/1.8.

A few different combination of aperture, shutter speed and ISO give varying results.

f/4 @ 1 second with ISO200. Good aperture and low noise but terrible motion blur.

f/1.8 @ 1/60s with ISO3200. No motion blur at all, but lots of noise and some wide-aperture softness.

f/1.8 @ 1/15 with ISO800. Balancing some barely noticeable motion blur, aperture softness and some noise.
When you have little light, some compromises must be made.

Don't be afraid to experiment and make sure you look over the pictures (and mind the settings they were shot at) and recognize which kind of blur you have for each unsharp image. If you don't know why your images are not sharp, the chances are you will keep getting bad results. Understanding the consequences of these decisions in low light conditions allows you to make the right compromise to get the most out of a tough light.

*The built in flash is usually quite terrible. If you can't get enough light by compromising the aperture, shutter speed and ISO then use it, but don't expect it to be pretty. If you have an external flash or any other lighting equipment you should be using that instead of trying to max out your camera's limits under bad light.

If you missed the explanations on what and where aperture, shutter speed and ISO are, go back to part I: Proper Exposure

In the last part of the series I will discuss some situations in which we intentionally under/over expose, I will talk about other reasons to choose fast or slow shutter and talk about other gadgets like a tripod, the built in flash and the ND filter.

Comments are welcome.

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