Crop Factor question

A friend of mine just got a used Nikon DSLR (D70 I think) which has a crop factor of 1.5. She used to have a film camera and really likes working with a fast 50mm lens. If I am understanding correctly, if she puts that lens on her Nikon DSLR she will have the equivalent of a 75mm lens. So it follows that she should look at something like a 35mm lens and its equivalent will then be 52.5mm, which is what she wants. The question I have then is about the speed of the lens. She knows she wants a fast lens and I see many different 50mm lenses at f/1.8 and 35mm lenses that are listed as f/2.8. How do these numbers compare? Is the 35mm (52.5 equiv.) at f/2.8 as fast as a 50mm (75mm equiv.) f/1.8 lens?
This is going to be her primary lens and is used to a 50mm at f/1.4 from her film camera.

i would say get the 50 1.8 for one yes the aperture makes a difference

the 50mm f1.8 will let twice as much light in than the 35mm f2.8 some will know the full list of appeture's.
Rob.

An f/stop for any lens focal length permits as much (or as little) light as the same f/stop on another other focal length.

But I assume your question is more along the lines, "For a particular focal length, a certain f/stop is typically the fastest f/stop for that focal length. If fastest is f/1.4 for 50mm, which is a 'fast 35mm lens"...35mm f/1.4 or 35mm f/2 or 35mm f/2.8 ?"

Well, you CAN find 35mm f/1.4 lenses!

Ahh ok I see...but am I correct in thinking that if she got a 50mm it would be the equivalent of the 75mm? If she wants the same field of view as she is used to with the 50mm on a film camera then doesn't that mean she has to get the 35mm so she is seeing the same field of view?

I see everyone talking about their 50mm lens and I see Canon has some excellent 50mm lenses. Does this mean that if they are using cameras that are not full frame then there favorite lens is really an 80mm (based on the 1.6 crop factor of 350D/400D) ?

There should also be a Nikon version of the Sigma 30/1.4 EX that I can wholeheartedly endorse ... in the 400$ price range.
http://www.sigma4less.​com …SLR%2B%3D28SG30​F14NI%3D29

Best regards,
Andy

TMR Design,
Yes the 50mm lens on a 1.5 DSLR will be like a 75mm lens.
To get the same field of view of a 50mm on a FF or 35mm film camera, a 35mm focal length is needed.

I just checked the Nikon site. There is a 35mm f/1.4. Sigma has a 28mm f/1.8. I don't see anything faster and I don't see 35mm.

Yes, if she uses a 1.5 or 1.6 crop format dSLR (e.g. Nikon or Canon 350) and she likes the picture content offered in a 50mm lens on film SLR, she needs about 32-33mm lens on the APS-C camera. Shooting with 50mm lens on APS-C camera is equivalent to shooting with 75-80mm lens on the film SLR.

Everyone loves their 50mm because
1. it is handy for shooting portraits on dSLR
2. it is quite fast, for low available light shooting
3. the f/1.8 lens is dirt cheap!

Thanks Wilt

Robert, I have just read the thread quickly and did not see any direct answer to your question about apertures. The "crop factor" is merely a way of defining a difference in field of view based on how large the film frame or digital sensor happens to be, relative to a 24mm by 36mm frame in a conventional 35mm film camera. There is absolutely no change in the lens itself. That means that a lens' focal length does not change, and its apertures do not change. A f/1.4 max aperture will be f/1.4 regardless of what format camera body the lens is fastened to.

A "normal" lens, equivalent to a 50mm lens on a 35mm film body, would be about 33mm on the "1.5 crop factor" body. On a Canon APS-C DSLR (10D through 30D with the Rebels in between) a "normal" lens works out to about 31mm. If you wanted a fast lens that emulates a 50mm f/1.4 lens on a 35mm film camera, you would merely look for the appropriate focal length with the desired max aperture of f/1.4. Whether or not a particular manufacturer is going to have what you want available depends on their marketing group and what they have figured they could sell a lot of.

Thank you Skip. I am glad you cleared that up and now I believe I have much better understanding of how this all works. I am becoming very aware of the fact that fixed focal length lenses are faster that their equivalent focal lengths in zoom lenses.

Correct me if I am wrong but I would guess that you have to be more creative and better at composition to shoot with a fixed focal length lens as opposed to a zoom.

Not necessarily better or more creative... it's just different. You need to physically move toward or away from the subject instead of zooming; the relationships of objects in the frame change differently when zoomed vs. moving in/out. Likewise, depth of focus isn't the same when zoomed in from a distance, compared to moving closer to an object. I fear today we take the "easy" way out and just re-frame a shot with the zoom instead of exploring the possibilities of moving in or out from the subject, and seeing how the spatial relationships change. Watch movies or TV programs with an eye to whether the camera is zooming, or rolling in/out on a dolly shot... there's a different feel to each technique.

I don't know if I would put it quite that way. It sort of depends on the style of shooting and the available "prime" (fixed focal length) lenses one has. I did an awful lot of my work with four lenses to stick on my Nikon F bodies. I used 28mm, 50mm, 105mm, and 300mm lenses. With those I never really felt I was missing much. Of course, in the old days zooms were nowhere as good as today's high end zooms.

As for the creativity and composition thing, I look at composition in a way that is perhaps different from a lot of folks. I first try to find the location for me and my camera to work from that gets the subject perspective and position to be what I envision for my image. Then, I choose a focal length to fill the frame as much as possible with the subject. It's easier to get the "right" focal length when I have a collection of zooms, but when using primes I would only have to crop an image now and again to make the final photo come out the way I wanted it to. I still crop occasionally, because the 2x3 ratio of the camera's output is not always correct for a given image.

The biggest advantage to today's primes over the zooms (I'm talking about high end lenses for both types) is the larger max apertures available in the primes. Size and weight are also smaller for the primes. However, I choose to use a trio of zooms with my latest setup. They are fast enough, but far more flexible than a collection of primes. They also weigh enough to give me some good exercise.

I'm glad Skip cleared that up-no one else semed to be addressing the aperture question.

I'll put it another way in case it helps anyone else understand .....
Aperture numbers don't change when using a full frame lens on a 1.6 crop camera -the reason for this is because the aperture number is an 'absolute' light measurement.Or stated another way -it's not a measure of light per area.Chopping the edges off the image (as 1.6/1.5 crop cameras do) doesn't reduce the amount of light the lens lets through