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02-14-2011, 02:08 PM   #1
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lens mm markings, crop factors, and fisheyes...?

Hi everyone,

I'm looking to purchase a new lens and I'm starting to get a bit confused at what I'm looking at...so please HELP!!!!

I know that when using a dual film-digital lens or digital only lens, there is a crop factor to take into account with regard to use on the APS-C sensor (1.52x factor). So for instance, any of these lenses marked 50mm would have a field of view of approximately 76mm in actual use on the APS-C sensor.

I know that when dealing with a fisheye lens that was originally based for film-digital, most of the fisheye effect will be cropped out. What about the newer fisheye lenses, such as Samyang/Rokinon/Bower which are being offered for APS-C sensors only (digital only)? Will they keep the full fisheye effect intact or will it be cropped on these lenses as well?

So, for example, will the Samyang 8mm FE then be a 12mm FE showing the full diagonal FE effect on the APS-C sensor?

Thanks for any light you can shed on this!

02-14-2011, 02:13 PM   #2
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you need to forget about crop factor all together, and consider field of view only in degrees.

the samyang 8mm fisheye has a 180 degree field of view (corner to corner, as that is how FOV is specified) on an ASP-C sensor, and in fact it is not speficied for full frame or film use.
02-14-2011, 07:20 PM   #3
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Lowell is quite right. Note that various types of 'fisheye' and ultrawide projections exist, so don't expect all 'fisheyes' of the same focal length to have the same coverage, even if intended for the same frame size. And some lenses that are fisheyes on FF are considerably less fishy on APS-C, such as my Vemar 12/8 and Zenitar 16/2.8. So when considering any lenses, just compare the AOV (angle of view) of each.

Something you *won't* get with these lenses is a 180-degree full-circle image. The Samyang 8mm or the DA10-17@10mm may be 180 degrees corner-to-corner, but they use frame-filling projections. On APS-C, a full-circle fisheye needs be closer to 5mm, and that's a pretty rare fish. That can be approximated -- I have an old huge Kenko 180 Degree Fisheye adapter that I mount on a 35-70 or 28-80 zoom. Depending on the exact lens, I get a full-circle image with the zoom around 37-42mm. The image fills the frame at around 60mm, and around 80mm has coverage similar to the DA10-17@17mm. But edge sharpness is lost with that adapter.

We'll talk about defishing another time.
02-14-2011, 09:33 PM   #4
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QuoteOriginally posted by RioRico Quote

We'll talk about defishing another time.
Don't get ahead of me....

OK...I think I follow. But then why the XX mm delineation? Isn't 180 degrees, 180 degrees...or am I being naive?

I'm looking at a few different things with regards to a new/used purchase. I want something fairly wide as a prime. It will be used mainly for landscape work and travel photography. I've been looking at the different fisheye shots posted around the lens forums and like the results. I see a fisheye as a lens that can be fairly versatile in my kit...something fun as well as serious...with or without being defished.

I've been looking at the Zenitar 16mm FE...but there's the metering issue (no green button on a K100ds). The Bower 8mm FE has metering but then there's the issue with no auto focus (as well with Zenitar). I know with wide lenses it's not that big a deal, but with no split-image screen...it can still be a headache. I've also located two Sigma 15mm/2.8 EX FE's with auto everything (not the latest DG version). I've read some good things here about them. The question is, how much money do I want to spend for a wide-angle prime.

02-15-2011, 08:28 AM   #5
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I don't know what the Zenitar seels for right now but I do know that prices really increased. That fact paired with the fact that the newer Samyang (or one of its other named sibling) is way better (supported by the Samyang 8mm in my bag...) makes me suggest the Samyang over the Zenitar and the Sigma because of its selling point.

For an once in a while lens, even the ~240/322$ are too much.
But the lens is great and I can assure you that focusing is a joke - meaning that you essentially don't even have to think about it - DOF is huuuge.
Ok, I have a split screen installed but even without it, focusing would be no issue at all. Only con of that lens I've found at all is its near focus limit. 0,3m/1yd isn't that close for 8mm.

And to answer your other question: as you know, focal length remains the same one every format but FOV doesn't. So 180 aren't 180 on another format. Can get tricky if you are dealing with a "full frame" lens because often statistics are given for this very format, meaning that you can't carry them over to your Pentax DSLR (you may already know all of this but I can't say so I'm writing it nevertheless...).
02-15-2011, 09:45 AM   #6
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You don't need autofocus on Samyang 8mm FE. When set to 0.4m and with f/9 aperture, it is sharp to infinity. I have determined these parameters experimentally as most sharp all over the frame on my sample so I usually only use them.

Also, the minimum focus distance is pretty far so you won't do any close ups anyway...

The thing to understand about the difference between Samyang fisheye and other fisheyes is that Samyang is much more suitable for panorama/landscape shots, when you want to capture the most details of the surroundings, while traditional FEs are good if you want to focus on something specific.

This is because Samyang projects nearly equal proportions for everything in the frame, regardless of the position of the object on the frame. So objects appear equally small everywhere on the frame (in fact, a little smaller in the center, but not much). So you can use these images to obtain de-fished picture of any part of the image and receive approximately same details.

Traditional fisheyes, however, are projecting increased proportions of the central area of the image so the objects appear more compressed the farther they are from the frame center. This means that the objects in the center appear much larger than on Samyang 8mm images, which is good for special effect shots and close up shots, but bad for panorama/landscape shots. If you'd want to defish such image, already compressed edges would be stretched to their live size so you'd get terrible resolution everywhere but in the frame central area.

Also, traditional fisheyes have pretty close minimal focus distance which enables semi-macro work - like Pentax 10-17mm fisheye.

By the way, there is Sigma 10mm diagonal APS/C fisheye and I believe also another circular fisheye (also for APS/C) but they are pretty expensive.

Sigma 15mm FE is FF so on APS/C you'll get distorted wide angle image instead of actual fisheye - like the long end of Pentax 10-17 FE zoom.

Last edited by olenl; 02-15-2011 at 09:59 AM.
02-15-2011, 11:31 PM   #7
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QuoteOriginally posted by dwhopson Quote
But then why the XX mm delineation? Isn't 180 degrees, 180 degrees...or am I being naive?
The XXmm designation is the focal length of the lens at infinity. It is only indirectly related to FOV.

Fisheye lenses differ from the regular rectilinear wide angle in that they utilize one of several forms of circular projection. Paradoxically, this results in less distortion (relative size of objects are appropriate regardless of relation to the lens axis) despite the "bending" of off-axis straight lines. For a given focal length, a fisheye will generally have somewhat wider FOV than a rectilinear lens.

As for 180 degrees being 180 degrees...it all depends on the media size and how you determine the FOV. All 180 degree fisheye lenses that completely fill the image frame measure 180 degrees diagonal across the frame for which they were designed. Lenses that project a circular image into the center of the frame have a true 180 degree FOV for all directions. Lenses made for larger media will generate a cropped image on smaller media.

Here is an example of why you have to be careful even with 180 degree diagonal:



The camera was level. Notice the tripod leg at the bottom of the frame!

Fisheye lenses do not offer intrinsically closer focus, nor do they have greater DOF for a given image size on the sensor. In fact, focus may be a huge issue with these lenses due to a tendency towards short focus throw. My Zenitar 16/2.8 is probably the most difficult lens in my quiver to accurately focus in the 6-10 foot range.

As for fishiness when using a FF fisheye on APS-C...It all depends on the subject and camera attitude. I have taken many landscape photos with the Zenitar on my K10D that have a very normal appearance. I have also done a few that look very fisheye. The trick is to leverage near-far relationship and/or shoot with the film plane in a non-plumb orientation. Tilt the camera up/down and the horizon bends in proportion to the tilt. Placing straight lines off-axis is another good trick.

For examples both with 35mm film and APS-C digital, here is a selection using the Zenitar from my Flickr account:
Flickr: Search fotostevia's photostream

Steve

Last edited by stevebrot; 02-15-2011 at 11:59 PM.
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