[metropolis4]

So I'm a bit confused about the way aperture values work on a DSLR. Are the numbers equivalent to 35mm format?

Why does the largest aperture available get smaller as you zoom?

Why is the maximum aperture on all the kit lenses so small? It seems like most of them don't go beyond f3.5 at the widest angle of the lense. Is that just with the cheaper lenses?

I'm so used to being able to shoot at f1.4 and f1.8 with a 50mm lense. Can you only do that with primes on a DSLR?

[redbaron]

the largest apeture value has to do with the lens, not the camera body.

on a lens that's say f/3.5-5.6 that means that at its shortest length wide open is f/3.5, but at it's longest focal length, wide open is f/5.6. Usually this is a compromise between making a lightweight affordable lens. Canon & Nikon both make a 70-200 F/2.8 for instance that is 2.8 wide open at both ends of the zoom spectrum, but it's also ~$1700.

You can get primes for a DSLR depending on what your body is & what is available. Nikon & canon both have a 50 1.8 and a 50 1.4. I would consider looking for something like a 28 or 35mm prime though if you want a 50mm replacement, since crop factor + 50mm lens tends to make it fairly limited in usefulness IMO.

Other than that, small f-stops (wide open) are going to give you very shallow depth of field, and big numbers (stopped all the way down) will give you almost infinite depth of field at the compromise of making a very flat image, the same way a film camera would.

I've found that on most of the mid range lenses, the best looking/sharpest tends to be somewhere around f/8 or f/9 depending, but usually in that range, or within a few steps.

[Rainer.]

Originally Posted by metropolis4

So I'm a bit confused about the way aperture values work on a DSLR. Are the numbers equivalent to 35mm format?

Yes.

Modern SLRs, including DSLRs have given up the hard stop system based on powers of two in favor of a more fluid range of apertures and shutter speeds still based on a log scale. Good for folks who know what they are doing, but a bit unfortunate for learning how the stops system works.

But 1/125th at f/8 is still going to be the same on either a DSLR or film SLR.

Why does the largest aperture available get smaller as you zoom?

As redbaron said, a compromise between cost, size, and manufacturing.

The maximum size of the aperture is determined by the size of the opening light goes through, of course. But, due to the optical physics of the machine, the size the lens has to be at the far end in order to achieve the same aperture value increases greatly with longer lens lengths. The path the light takes in order to reach the film looks like this: > . So, you can see how you need very large elements at the end of a very long lens in order to maintain a wide aperture.

Now here are two f/2.8 primes:
20mm:

And compare it to this 400mm lens:


Now, in a compact zoom lens, the diameter of the lens is relatively small, so when you zoom further, the effective aperture of the lens also gets smaller. For example, this 55-200 f/4-5.6:

With lenses with much larger front elements, it becomes possible to keep a large aperture throughout the zoom range, for example this 70-200 f/2.8:


Essentially equivalent focal length ranges, but you can see why the casual shooter taking pictures on the family vacation probably wouldn't want to have the second lens.

I'm so used to being able to shoot at f1.4 and f1.8 with a 50mm lense. Can you only do that with primes on a DSLR?

Zoom lenses (for all cameras, DSLR or 35mm SLR) are generally manufactured at max apertures of 2.8 (very expensive, again). And consumer models (under $1K) generally top out at 3.5 or 4.

Prime lenses are much simpler. And so can be manufactured to have much larger apertures for far cheaper. I use a 50mm f/1.8 that I picked up real cheap on my Nikons.

[ApparentlyNothing]

Originally Posted by Rainer.

Zoom lenses (for all cameras, DSLR or 35mm SLR) are generally manufactured at max apertures of 2.8 (very expensive, again). And consumer models (under $1K) generally top out at 3.5 or 4.

I got a Tamron 17-50mm that's f2.8 throughout in the neighborhood of $400-500. And it's super sharp, even at 2.8. So there's definitely sub $1000 zoom lenses with consistent 2.8 aperture that are actually good.

[Rainer.]

Originally Posted by ApparentlyNothing

I got a Tamron 17-50mm that's f2.8 throughout in the neighborhood of $400-500. And it's super sharp, even at 2.8. So there's definitely sub $1000 zoom lenses with consistent 2.8 aperture that are actually good.

Oh, nice. I may have to look into one of those...

[ApparentlyNothing]

Originally Posted by Rainer.

Oh, nice. I may have to look into one of those...

Yeah, not sure if the Nikon version is just as good, but I would assume it would be.

[metropolis4]

Ok, that makes sense. I was confused because I have two old zooms for 35mm. One is 75-150 and the other is 60-200 and both are (supposedly) the same aperture throughout the length of the zoom and they aren't nearly as wide barreled as those zooms.

I was mostly confused about the shorter zooms like the 18-55mm kit lenses. At 55mm the best they will do is f5.6 which seems crazy. I'm used to having at least one stop if not two below that at 50mm.

I really like bokeh, and blurring backgrounds to isolate focus is sort of a new discovery that I'm having a lot of fun with, and it seems like you'd have a more difficult time doing that when you can't shoot wider than f5.6...

On a side note I did try a Nikon I liked! The D90 seemed really nice to me. The only thing I could find I didn't like (which a lot of other camera's do also) is the position of the dial on the back. I kept hitting myself in the eye while trying to adjust shutter speed.

I also checked out a Canon 50d that was nice, but everything looked way overdeveloped in the LCD... I'm not sure what was going on there.

But anyway, not to derail my own thread, I guess I'll have to keep looking at lense options before making a decision on a camera...

[tht00]

Originally Posted by metropolis4

I really like bokeh, and blurring backgrounds to isolate focus is sort of a new discovery that I'm having a lot of fun with, and it seems like you'd have a more difficult time doing that when you can't shoot wider than f5.6...

Me too. I have a Rokkor 50mm f1.4 that I love and so when I went digital, I picked up a Nikkor 35mm f1.8 (~52mm equivalent) and it's a nice lens. Bokeh isn't quite as nice -- I can't blur the background into oblivion as I could with the 50mm, but the DoF seems sufficiently shallow for most things. But I don't have any keepers from this lens yet or I would put them on flickr...

I can get bokeh with my 18-55 f3.5-5.6 lens kit, but I either have to be sufficiently close (often unusably close -- it can focus ~5 in), or use a higher focal length. Even though the f-number gets smaller as you zoom, the DoF gets smaller and bokeh becomes more apparent. I believe this has much to do with focal length, which is probably heavily related to the circle of confusion for determining focus or lack thereof. But this aspect of lens attributes I'm only vaguely familiar with.