Fixed HQ lens with image crop or medium quality zoom lens?

Question

Is it better to spend my money on a medium quality say 15-85 zoom lens or a very high quality fixed lens at say 24MM. (e.g 24mm 1.4 L canon) and crop in - not blow up beyond normal resolution).

A couple of points to consider for my situation:

1. 90% of my images are 12x18 or smaller - down to 5x7.
2. I only do B&W prints on matte paper at 200 dpi.
3. All of my shots are nature/landscapes shot from a tripod (no animals, action, etc.)
4. I'm using a Canon 7D at 18Mp raw.
5. I don't need IS and probably not AF.
6. I would find an f-stop / range guide very helpful for focus (old time 5x7 shooter).

It seems to me that a very high quality L and image crop would yield a better image than a medium quality (lets say same price) zoom. But then I really don't know. The biggest problem is knowing the final image crop with one image in the viewer.

Your thoughts appreciated.

TIA

Answer 1

With landscapes you are going to be shooting at fairly small apertures so a medium range zoom will do the job very well actually. Even cheap lenses are sharp when used at the optimum aperture (usually f/5.6 - f/8). What you pay for with the 24 f/1.4 among other things is the very wide max aperture, which you don't really need. It may be sharper at say f/2.8 but you are going to lose the advantage quite quickly when you start cropping.

Answer 2

How often do you use the various focal lengths? It can be instructive to review the photos you have taken. If you use the longer lengths any appreciable amount of time, you will miss the zoom: cropping a 24mm photo to get the equivalent of 85mm reduces your effective pixel count to 18 Mp * (24/85)^2 = 1.4 Mp. That's pretty crummy even for the smaller prints you contemplate.

We can run this formula in reverse, too: a 12 x 18 print at 200 dpi is 2400 * 3600 = 8.64 million dots. This means that if you have an incredibly sharp lens (so that its image quality is limited by the sensor resolution) you can afford to crop by a factor of Sqrt(18 Mp / 8.64) = 1.44. This means you might just be willing to use the 24 mm glass for a shot that would be exactly framed by a 24 * 1.44 = 35 mm lens. In practice, though, even the best lenses do not resolve at the pixel level on an 18 Mp sensor, so you're going to notice a bit of degradation anyway. If, perchance, you had a pair of primes--say, the 24 mm L and the F1.4 (or even F1.8) 50 mm lens, which are relatively (respectively, dirt) cheap, then you would have moderately good coverage in the 24 - roughly 72 mm range between them, with a gap around 35 - 50 mm that conceivably could be filled in later with a 35 mm prime. That zoom covers a lot of ground!

Answer 3

It's highly variable. If you really wanted to know, you'd have to look at the MTF charts and other information for both lenses at various focal lengths, apertures, focus distances, diffraction limits, crop amount, and sensor resolution limits. Note that lens softness and cropping look different. Optical softness will be the overlapping of airy disks and will look much different than the eventually visible blockyness that will result in cropping, which can be more distracting.

A zoom lens at the $1500 range will not be a "medium quality". You can get some wonderful L zooms in that range. Cropping a 24mm lens to 85mm equivalent will be very low resolution (the equivalent of taking a picture with a much smaller sensor) and would even begin to compare.

The price of that prime lens is for its size, weight, and more importantly speed and the bokeh that comes with it. Given your scenarios, I don't believe you will be taking advantage of those and you are much better off with a zoom. At the f-stops you will presumably be using for deeper DoF, the difference in IQ becomes more insignificant. The zooms may not have the depth of field guides, but you can learn hyper-focal distances.

Answer 4

While a prime lens would offer some slightly better corner quality than a zoom lens, you might not want to underestimate the value of a wide angle zoom on such a high resolution camera body. It should also be noted that a lot of the high cost of Canon L-series prime lenses is due to their very high speed and accurate AF. You may find more value in investing your money in a zoom lens with ok AF rather than a prime with superb AF.

I use the Canon EF 16-35mm f/2.8 L II lens myself, and it is an amazing lens. Sharpness and clarity are excellent throughout its focal range. There is some softness in the corners at the extremes on a full-frame body, but on a cropped sensor like the 7D, a lot of that is eliminated. The focal range of the EF 16-35mm is effectively 25-56mm on the 7D's sensor, and costs about the same as the 24mm L II prime. Optically, the 24mm is better, however a considerable portion of its cost is due to the very high speed, advanced AF...a feature you mentioned you did not need. From a cost benefit perspective, you would get more out of the 16-35mm.

Alternatively, you could look into the EF-S 10-22mm lens. This would give you true ultra-wide angle on an APS-C body, effectively equivalent to 16-35mm. Optically it is inferior to both the 16-35mm L II and the 24mm L II, however it is a lot cheaper than either. It is a slower lens, but for landscape work, that may not matter. The key factor here would be image quality, and sharpness is a bit lacking with this lens.

With a prime lens, you would be limited, and would have no choice but to crop in post processing to get exactly the framing you want. With a zoom lens, you can frame in the field and maximize the potential of your shots. For the size of prints you use at the resolution you print at, a single shot at 18mp that is properly framed in the field could be used to crop out numerous interesting prints (one of the benefits of having a high megapixel sensor.)