Monday, October 29, 2007

Getting sharp photos in low light

The gorilla's hand

(Click on the photo for a larger version, the EXIF data, etc)

On Saturday I met Mike (ndisgr8) from my yahoogroup at Brookfield Zoo and we spent the whole day taking pictures. It was overcast in the morning, which made many of the indoor exhibits, which are largely lit up via skylights, very dark. As a result, it was extremely difficult to get high enough shutter speeds to get a decent photo in there. I figured this was a great example of low-light photography which everyone seems to run into as a problem from time to time, so here's a write-up about how I dealt with the situation and managed at least a couple decent pics from the ape house...

I was using my Sigma 50-500mm f/4-6.3 lens because this zoo gives the apes a lot of space to roam around and play, so I needed all the "zoom" I could get. For those who aren't familiar with the "f numbers" on lenses, it's included in the description of the lens for just this purpose (low light photography). The lower the "f numbers", the better the lens performs in low light. When you have a range of f-numbers like for this lens, it means that at the wide end (50mm) , 4 is as low of a number as you'll be able to choose for your aperture. At the long end (500mm), it's 6.3.

The reason this matters is as follows: The lower the aperture number, the faster your shutter speed can be. This is because as the aperture numbers get lower, you're letting more light into the camera, with more light via aperture, you can get faster shutter speeds (which allow less light to get in because the shutter doesn't stay open very long). -It's all a matter of balance.

Step 1: Aperture
This lens (the "Bigma") is not very "fast" (meaning it has relatively high f-numbers), so it really limits me when there's not much light. Because of this, I immediately put my camera on aperture priority mode and set it for it's lowest aperture number so I could get the most light possible via aperture (which would, therefore, allow the fastest shutter speeds possible).

Step 2: ISO
Before even bothering to begin looking for photo opportunities, I also set my ISO to 800. The higher your ISO number, the faster your camera will record the light when you take a picture. So, in low light you want to choose a high numbered ISO.

The cost for using high ISO numbers, however, is noise. Most noise can be removed in software later, but there's always an effect on picture quality. You should, therefore, always shoot in the lowest possible ISO setting for the lighting conditions you are in. In other words, remember to set it back down to 100 when you're shooting in daylight again.

Each camera is different as far as how much noise it produces at different ISO's, so it's a good idea to experiment and know how your camera behaves and how well you can remove the noise it produces at each different setting. This way, you'll know not to go above (in my case ISO 800) or you're risking not being able to remove the noise from the photo effectively. -I did end up moving to ISO 1600 later on in the ape house as a last resort because I wasn't getting any clear shots at ISO 800.

Step 3: Check your shutter speeds
Next, I lined up a shot and checked my shutter speed. With most SLR's you can see the shutter speed in your viewfinder. In this case, I was seeing shutter speeds ranging from 1/10 to 1/60 most of the time.

Remember, the rule of thumb is that you need a tripod if your shutter speeds are faster then the reciprocal of your focal length. That's just a fancy way of saying that if you're shooting with a 500mm lens like I was, your shutter speeds should be faster than 1/500th of a second. If you're shooting at 300mm, they should be faster than 1/300th of a second, etc. (Although 1/60th of a second is as slow as you should go with any lens because it's hard to hold the camera steady enough when your shutter speeds are slower than that regardless of how wide the lens is.)

So, I needed shutter speeds that were up to 50 times what I was getting!

Step 4: Put your camera on a tripod
I had a monopod with me just because my lens is so heavy, but I didn't have a tripod. So, I leaned the monopod against the railing and steadied it further by bracing it against side-to-side movement with my foot. A tripod would have made it more steady.

Of course, the monkeys and apes were rarely perfectly still, so even with a tripod, you'd still get blur unless they happened to stay still during the shot.

Step 5: Use burst mode
If you've done all you can with settings and steadying the camera, but your shots are still coming out blurry, your best bet is to take as many shots as possible. Many people aren't aware of it, but they jerk the camera a bit when pushing the shutter button to take a photo. That may be contributing to the blur, so if you put your camera in burst mode you can hold the shutter button down and it'll take several shots one after the other. Even if you're sure you don't jerk the camera, just by shear volume, this will increase your chances at a clear shot.

Step 6: Get creative!
I was doing all of that as well as trying to concentrate only on the apes that were in the best light and I was still having trouble getting clear shots. So, for a few shots I decided to try something different. I was shooting in RAW, so I knew that I'd have a good chance of getting most of the data out of a photo even if it was severely under-exposed (dark), so I intentionally under-exposed several photos as a last resort.

Your camera should have an exposure meter that looks like this: [-2...-1...0..+1..+2] and there's a small indicator that shows you your current exposure setting. "Zero" is normal exposure, but if you use exposure compensation to move the indicator towards the positive numbers, you're telling the camera to over-expose the photo by that amount. When you move the indicator towards the negative numbers, you're telling it to make the photo darker (under-exposed).

Darker photos are a result of letting less light into the camera, so when you move the exposure down into the negative numbers you should get higher shutter speeds. When you're shooting in RAW you're often able to lighten up the photo by up to 2 stops (which correlate with the numbers on that exposure graph on your camera).

So, I moved the exposure meter down to -2 and took a shot. I reviewed the photo on my LCD and turned on the option to view the image's histogram. I made sure the data didn't run off either side of the graph (because that indicates that you're missing data, which you can't get back even in RAW). The photo was very dark, but the histogram looked ok, so I tried a few shots like that and hoped I'd be able to "save them in software" later.

The photo above is one of the ones I took on ISO 1600 and under-exposed by 2 stops, then "saved" in the RAW software that came with my camera (Digital Professional Pro). The "right" solution to this kind of a problem is a "faster" lens (meaning a lens with lower f-numbers), but everyone eventually finds themselves in a situation where they didn't expect to have a low-light issue and they need to do what they can with what they have. Hopefully, this write up has given you some ideas for the next time you're shooting in low-light.

Extended reading / other resources:

Monday, October 22, 2007

Choosing a normal prime lens for a Canon mount.

I just purchased a new normal prime lens for my Canon 30D and it took a lot of research to decide which one to get because there are a ton of them on the market ranging from $75 to $1300 or more. If you're in the market for such a lens, I hope my research can benefit you as well.

First, let's define the terms "normal" and "prime" for those who may not be "hip" to the "lingo" (I'm cool like that).

"Normal" lenses are usually 50mm lenses. This is the sweet spot between wide angle lenses and telephoto, hence, they're called "normal". There's one caveat these days, though, and that's the crop factor of your camera's sensor. Make sure you take that into effect. If you have a cropped sensor Canon camera, you'll most likely be looking for a 28mm - 30mm lens in order to get as close to normal as possible.

"Prime" means the same thing as "fixed" and all that means is that it's not a "zoom" lens. You can't zoom in or out with it. It has just one focal length, so you have to "zoom with your legs". Prime or fixed lenses are usually cheaper and sharper because they are more simple to manufacture and compromises don't have to be made so that they can work at sometimes dramatically different focal lengths (I have a 50-500mm lens, for example!).

The research:
There are many options out there for Canon mounts. If you're looking for a 50mm prime there's 3 main choices including the Canon 50mm f/1.8 which goes for $75 or so new! It's surprisingly sharp as well and many people are very happy with this lens. The downside seems to be bad bokeh (background blur) and the build quality isn't great (it's fragile and very plastic-y). Still, for $75, it'd be a nice first prime if you're just wondering what they are all about!

50mm choices include:
  • Canon EF 50mm f/1.2 USM L
  • Canon EF 50mm f/1.4 USM
  • Canon EF 50mm f/2.5 Macro
  • Canon EF 50mm f/1.8 II
30mm / 35mm choices:
  • Sigma AF 30mm f/1.4 EX HSM DC (EF-S mount)
  • Canon EF 35mm f/2.0
28mm / 24mm choices:
  • Canon EF 28mm f/1.8 USM
  • Sigma 28mm f/1.8
  • Canon 28mm f/2.8
  • Canon EF 24mm f/1.4L USM
  • Canon EF 24mm f/2.8
  • Sigma 24mm f/1.8 EX Aspherical DG DF Macro
Prices, MTF Sharpness test results, Vignet or light falloff test results, chromatic aberration test results, distortion, SQF ratings, plus comparisons, sample images, lens tests, and lens reviews can all be seen side by side here in my online spreadsheet:

And for those who are interested, I ended up purchasing the Sigma AF 30mm f/1.4 EX HSM DC -I'll post a review soon!

Cropped sensor cameras -what's a "cropped sensor" and what effect does it have?

What's a crop factor?
Most film cameras are 35mm, and that refers to the size of the recorded image on the film. Professional-level ($$$) digital cameras have "full frame" sensors that are 35mm, and therefore replicate film exactly. So-called "Pro-sumer" DSLRs usually have smaller sensors in them because they are cheaper to manufacture. The smaller sensor obviously doesn't impact the image resolution much (DSLR's with cropped sensors are now being made at 10MP or higher resolution), but they do impact the way your lenses work.

Why might I want a full frame camera?
Full frame cameras have better high ISO performance.  This means, in low light when you have to increase the ISO to get a clear shot, you'll get less noise (that grainy, static-looking stuff in your photo) with a full frame than you will with a cropped sensor camera.  You may get a higher quality image as well, but you won't be able to tell the difference at normal print (11x14) or on-screen sizes.  As person who tries to buy the best all the time I held out for a full frame camera for a long time before deciding to go cropped instead.  I've found I have no need for a full frame and feel this is true for anyone who's not making enough money from their photography to cushion the extra expense.  I've sold photos taken with a point and shoot, so buying a crop sensor camera won't impact most sales if that's your aim.

How do we account for the crop factor when looking into buying a new lens?
Easy! Look up the crop factor of your camera. For most Canons (20D, 30D, 40D, 300D, 350D, 400D, Rebel, Rebel XT, and the Rebel XTi) it's 1.6. For other camera types, just google your camera's make and model along with the words "crop factor" and you'll probably get an answer right away.

Then all you have to do is multiply your crop factor times the focal length of the lens to figure out what the equivalent would be for film. So, if you're looking at a 50mm lens and you have a Canon 30D, it would act like a 80mm lens (50 x 1.6 = 80).

But if, instead, you're looking for a lens that ACTS like it's 50mm on your cropped sensor, then all you have to do is divide 50mm by your crop factor. 50 / 1.6 = 31.25. So something in the 30mm range would work out best.

Why does this matter?Most of the time it doesn't. You just use your lenses to get the photos you want. If your 100mm lens isn't long enough for you to photograph birds, you go look for a lens with a higher focal length, and you'll know that a 200mm lens will get you twice as close. None of that changes. And don't fear that your photos will be "cropped" from what you see in the viewfinder -they will not. What you see is still what you get. The only time this will matter is if you're getting advice from someone with a film or full sized sensor camera -or, if you're thinking of buying a full frame camera vs a crop. That's all no biggie :-)