focal lengths, crop sensors, myth vs reality (halp!)

[Irru]

In ~15 days time, I am buying a Pentax Kr and a selection of lenses. I'm relatively new to slr photography, though I have owned a Canon G10 for a while now, and was also recently the proud owner of a Panasonic GF1 for three weeks before it was stolen. (Woo travel insurance).

I'm trying to plan my initial lens selection, and I have come across a statement on the internet that confuses me, and may cause me to re-evaluate a few things once it is explained (assuming it is even true, but it seems to have credibility.)

I found it while researching what focal length I want in a fast prime to be used for indoor volleyball shots when I get back to Sydney next year. On the steve's Digital Camera forum, I found this:


50mm = 15 feet
85mm = 25 feet
100mm = 30 feet
135mm = not familiar with
200mm = 75 feet / 25 yards
300mm = 40 yards
400mm = 50-60 yards

Min distance depends on the camera - full frame obviously you can get more of your subject in the frame. And contrary to myth, using a 1.6x sensor camera doesn't really allow you to shoot from further away. So shooting these short primes on a 1.6 can be a bit problematic.

From my own rudimentary testing (I don't have a volleyball court nearby, but I played for 15 years so I just estimated a distance from a random person, and tried a few focal lengths until I found one that made the person more or less fill the frame) 50mm is what I want.

But what is the deal with that line:
using a 1.6x sensor camera doesn't really allow you to shoot from further away.

How doesn't it? Someone else in the thread mentions using 85mm for volleyball shots, but don't indicate whether they use ff or crop, or even how close to the sideline they are able to get.

I almost wish I hadn't read the thread - I was nicely confident that the 50mm is what I wanted, but now I am worried that I will buy something I never use.

[milesy]

because some people beleive that using a crop sensor increases the zoom of your camera / lens - when in actual fact it just means that the field of view is less - but a 50mm lens is still only 50mm

[Cage]

It is really all about FOV (Field of View) and subject distance, or focal length.

An average 50mm lens on a 1.5 crop sensor has a FOV of around 32 degrees and 100mm is around 16 degrees.

What you need to do is work out how much you want in the frame at your available shooting distance to decide which lens is suitable.

[Irru]

Ah right. So it comes down to 'perspective' rather than purely 'does my subject fill my field of view or not'? Tell me if I have this right:

Picture of person, head at top of pic, feet at bottom:
FullF: 50mm 'normal perspective'
Crop: 35mm 'wide perspective'
Crop: 50mm (stepped backward to fit subject) '________ perspective'
FullF: 75mm (stepped backward to fit subject) 'telephoto perspective'
FullF: 35mm (stepped forward to fit subject) 'wide perspective'

I think the missing word there, is... normal, right? You're saying that even though I am standing further away, it isn't telephoto? Or are you saying that it is only normal if I don't step back and the subject simply doesn't fit my picture?

Arg! This is hurting my head.
(I think the answer is that it is telephoto perspective, and I cannot figure out how I am wrong.)

Edit: Kevin, you posted while I was composing my post, so I didn't see yours. I think you confirmed my original understanding: don't think too much about it, just pick the lens that fits the subject.

[OzzieTraveller]

G'day irru

It does come down to Field / Angle of View ... .and what these days "equals" what a 35mm /full frame would see

maybe this qwik sketch will help ...
http://i51.tinypic.com/2cqie0o.jpg

You will see that for a given lens, the different sensors 'see' different FOVs - and thus it leads to the quote "this APS lens is the equivalent to that 35mm lens"

Hope this helps
Regards, Phil

[ricktas]

As Oztraveller says, it's the field of view that changes, not the zoom factor of the lens. A 50mm lens is a 50mm lens it doesnt magically change when it is put on a camera body.

Have a look at this : http://www.ausphotography.net.au/forum/showthread.php?36999-Learning-Centre-Appendix-D-Full-Frame-vs-APS-C-DX-crop-factor-w-example

[Kym]

maybe this qwik sketch will help ..
Snap! http://www.ausphotography.net.au/forum/showthread.php?36999-Learning-Centre-Appendix-D-Full-Frame-vs-APS-C-DX-crop-factor-w-example&p=613715&viewfull=1#post613715
(Same as Rick's above)

[Irru]

Thanks for the replies. I understand now.

Happy Christmas by the way.

[kiwi]

I'll save you the hassle

Brt a sigma 70-200 f2.8 hsm for sport of this kind

[Irru]

I'll save you the hassle

Brt a sigma 70-200 f2.8 hsm for sport of this kind
Thanks for the advice. It doesn't really work for me though; this is only a hobby. I won't be doing it seriously enough, nor often enough, to warrant the cost.

The 50mm is the right focal length for me. I sit 2-3 metres from the sideline, on the seat next to the coach. I can wander around the edge of the court, and even get a little closer if I need to. I've tested the focal length and am happy with that, except for the passage I read in that quote; the wording suggests that I cannot use that 50mm. That got me worried despite my own testing, but thankfully after reading this thread I understand that the whole thing is essentially just semantics. You pick the lens that fits the subject, period.

(The only exception might be when considering perspective from a wide or telephoto angle lens. I.e. a 35mm lens is wide angle, and even if you stick it on a crop sensor where it is equiv to 47mm, that doesn't change the fact that it is going to tend to look as though the subject is closer than if you were looking with the naked eye.)

[Cage]

Irru, I'd have a serious look at the new DA 18-135 and if you buy it as a kit with the Kr you should get a good deal.

Here is a review http://www.pentaxforums.com/reviews/pentax_18-135mm_review.php

[Irru]

Interesting idea. It is reviewed well. That suggestion actually changes a few things for me as it would cover a couple of the other uses I'm trying to plan a lens selection for.
I wonder if 135mm on crop sensor would be close enough for the water skiing shots I am after. Our ski rope is 70' long and 135*1.5 is 200mm.

Thanks for that suggestion, I'll spend a bit of time rethinking it, and researching focal lengths for waterskiing. I'm sure there will be some shots in the sports section here somewhere where the gear is listed.

[Cage]

If you want to get extra length also look at the DA 55-300mm. Seems well regarded too.

Cheers

PS: Check this link for some user reviews. http://www.pentaxforums.com/lensreviews/SMC-Pentax-DA-55-300mm-F4-5.8-Zoom-Lens.html

[TOM]

what the crop sensor does also is effectively give you a digital zoom - it puts more pixels in the 'crop'. so whilst a 200mm lens on a 12mp 36x24mm sensor will give you a wider FOV than that of a 12mp smaller sensor, if you crop your image from the bigger sensor, to equal the same crop as the smaller sensor, the smaller sensor will retain more detail because it will still be a 12mp image. The larger sensor's cropped image will have a significantly reduced image size. I had a crop mode on my D3, and from memory, the image size went from about 12mp down to around 7mp. The term "full frame" is a misnomer, as a 24x18mm or a 18x12mm sensor is also full frame. The 36x24mm digital sensor is a legacy of the film format, but has the most potential in small format digital.