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April 16, 2009 5:41 AM

What's the best way to compensate for the slight darkening effect produced by zoom lenses?

Encouraged by positive reviews, I picked up an 18-200 mm zoom lens for my Nikon D40 DSLR camera recently.

I realized that, given the amount of glass in front of my shutter, my pictures would appear slightly darker than those taken with the D40 kit lens. I'm now looking for workarounds for the darkening effect, both on-site as I'm taking the pictures and in the "post-production" phase of correcting with photoshop and iphoto.

Specifically, the darker or more saturated elements of subjects seem especially prone to appearing darker in finished photos, as do photos in which backlighting is present.

What should I keep in mind in the field to reduce the darkening effect of the zoom lens, and what tricks can I use in iPhoto or Photoshop to mitigate the issue?

posted by Gordion Knott to technology (15 comments total)

Are you using the camera's automatic exposure control? Have you tried increasing the exposure compensation when using the lens?
posted by odinsdream at 5:58 AM on April 16

(Or, decreasing... I guess.)
posted by odinsdream at 5:58 AM on April 16

odinsdream, it's on automatic, but I'll try adjusting exposure compensation.
posted by Gordion Knott at 6:12 AM on April 16

Your kit lens is probably a zoom too, if not as wide-ranging, right? Zooms per se don't darken images.

I'd speculate that the camera is underexposing slightly compared to what you saw with your shorter kit zoom in similar lighting circumstances, because it's trying to keep the exposure time short enough to avoid blurring from camera shake, which is more of a problem with longer focal lengths.

You can prove to yourself that the lens does not "darken" images due to having more glass, if you put it on a tripod and play with manual exposure, or shutter priority.

Long zooms like the one you bought typically don't have very wide apertures because of the high cost of manufacture. A range like 18-200mm is much more difficult than 18-35mm, for example.

The quality of the glass can effect colour saturation, sharpness, etc., in a way that looks a bit like dark/light. But I think your kit lens and the one you bought probably have comparable glass.
posted by galaksit at 6:22 AM on April 16

Yeah, try adjusting the exposure comp. to +.7 or +1.
posted by lockestockbarrel at 6:49 AM on April 16

TTL metering will give you equal exposures whether you have 4 elements or fourteen. The camera could be trying to compensate for the extended focal length by forcing an optimum shutter speed/aperture combination which doesn't work due to the lens already being at maximum aperture. I've seen this happen on crappier cameras and lenses.

You mention backlighting which obviously can screw up the image and confuse the TTL metering. Backlighting is best handled by spot metering of the subject or by separate handheld metering. It can be tricky because if you use average metering, it will make the subject darker. The camera can not be expected to output perfect photos all of the time on its own. Bad lighting abounds and even the best photographers need to work hard to get a good photo under these conditions. Novice photographers need to know when they are beat and probably just try to bracket the hell out of a shot to get something passable.
posted by JJ86 at 7:13 AM on April 16

Shoot in RAW then bump up the exposure in post. You usualy have 2-3 stops of EV both ways.
posted by gergtreble at 7:16 AM on April 16 [1 favorite]

There's absolutely no reason to change exposure compensation. Your camera's basic automatic exposure calculation will account for the fact that less light comes through the lens (higher f-stop). The overall picture should come out roughly the same brightness, subject to the vagaries of any specific scene. If you don't like the exposure then tweak it in Photoshop afterwards. You'll get better results if you start with RAW.

However, a wide range zoom like 18-200 will have a couple of other undesirable effects. One is vignetting, the edges of the photo will be darker than the middle. The other is chromatic aberration: you'll see little blue or red fringes on the edges of things, particularly in the corners of the photo. Both of those effects are correctable in post-processing. I use the free PTLens Photoshop plug-in, but there may well be better options.
posted by Nelson at 7:45 AM on April 16 [2 favorites]

Nelson: FYI PTLens is no longer free.
posted by fake at 7:50 AM on April 16

As others said, no matter how many elements your lens has, your camera ought to be compensating for this (perhaps adding the risk of shaky pictures or a decrease of depth of field, but that's another issue).

Tele shots usually need a more careful choice of what the camera actually uses for measuring the right exposure than, say, wide-angle landscapes or some such. If you would be zooming in on a relatively bright part, your exposure might automatically get messed up and the rest of the picture gets too dark.

I'm not familiar with this particular camera but if we assume that you're measuring exposure automatically, that your camera uses some kind of standard measure-most-in-the-middle-of-field setting and that you're specifically interested in shooting tele-shots, you might try to measure a balanced bit of your subject, including light and dark passages, hold the resulting exposure settings by using the designated button for this and recompose your picture after that.
posted by Namlit at 8:36 AM on April 16

As mentioned, shooting in RAW is nearly magical as far as the ability to adjust exposure after the shot. iPhoto is a great tool for this. You may also be stunned at what you can do by adjusting the individual sliders for shadow, midtone, and highlights in iPhoto.

But there really is no reason that a properly exposed shot with a long zoom should be any darker than with another lens, aside from the possibility of the corners having a slightly dark vignette, depending on how far you are zoomed out. (I'm not sure if that's an issue with your particular lens, but some long zooms have this issue.)

I wonder if the "darkness" is created by haze in the atmosphere, or by the contrast charateristics of the lens.

In any case, messing with your exposure compensation on the D40, experimenting with different types of metering, or getting out of full auto mode (I nearly always shoot in aperture priority), along with shooting in RAW and learning what you can do in iPhoto, might just give you better photos than you thought possible.
posted by The Deej at 9:38 AM on April 16

The quality of the glass can effect colour saturation, sharpness, etc.

Good grief. I meant affect of course. Can I blame pre-coffee posting? Go on then ...
posted by galaksit at 9:53 AM on April 16

Namlit pretty much nails my experience with the D40, particularly if you're not specifically using centre-weighted or spot metering.

I almost always do spot-metering and recompose as he suggests - unless I've got a lot of bright background (e.g. sky) which I want to keep from blowing out, in which case I use the full metering (aka "3D Colour Matrix Metering II" in Nikon parlance) and fill-flash; the onboard flash works quite well for this at shorter (< 20'~30') distances.

My experience with correcting for all this in post with RAW is that you never have quite enough headroom to fix it properly. Having said that, I've found that leaving exposure comp. at 0 for daytime shots, and upping it to +0.7 ~ +1.0 - or even higher on occasion - for early morning / late afternoon shots when the light isn't there and you want to widen the contrast range a bit, makes it pretty right straight out of the camera and gives enough leeway to touch things up in post.
posted by Pinback at 5:35 PM on April 16

I love my 18-200VR on a D50. I'm not sure what you mean by 'darker'. The lens is not super-great in low light, especially zoomed in a bit (F3.5-5.6), the VR helps a bit though. "I realized that, given the amount of glass in front of my shutter, my pictures would appear slightly darker than those taken with the D40 kit lens."
Have you actually seen darker images, or is this simple speculation? The D40 kit lens has the same f-value at 18mm as the 18-200 (kit lens 18-55mm f/3.5-5.6G), and cruising much faster up to 5.6. You won't be at f5.6 at 55mm with the 18-200, as it's a bigger lens (thus gathering more light).

The most important thing to note is that the 18-200 has a much larger front element (72mm filter size) vs. the 18-55, thus letting in more light. This will more than compensate for any (minimal) transmission losses due to more glass.
posted by defcom1 at 10:43 PM on April 16

Oh. And have I mentioned I love that lens. It's great. It's almost always on the camera. It's minor imperfections are far outweighed by it's versatility!
posted by defcom1 at 10:44 PM on April 16