[talkskiwon]

One thing I find curious about pentax camera and lenses as a beginner in photography is that many people use FF lenses on their crop sensor bodies. And, I believe there is no disadvantages of FF lens on crop-body because the image coming through the lens is perfectly covering the sensor and there is no problems on picture image. In fact, because FF lenses are shooting bigger images than the sensor itself to the sensor, the corner contrast and sharpness issues we see from APS-C lenses on APS-C body is not present on the FF lens on APS-C body. Does this make sense? and is there any disadvantages of using FF lens on crop bodies? I think this could help me making right choices for lens purchases. Thanks!

[stevebrot]

Sounds like you have it covered. Some claim that the extra light bouncing around the mirror box may result in a reduction in contrast, but that has been sort of hard to demonstrate.*


Steve

* Maybe someone with a K-1 and a K-5IIs will do a comparison series with the K-1 in crop mode.

[Kerrowdown]

is there any disadvantages of using FF lens on crop bodies?

None really as I already had lenses from my film days, so I've been using FF of crop for the last eight years or so with Pentax. The only disadvantage is that the lenses are usually bigger and heavier than their crop equivalents.

I think this could help me making right choices for lens purchases

If you will end up with a FF in the future, then perhaps that's the way to go, if you've no intention then crop may well suit you better.

[Na Horuk]

Sounds like you have it covered. Some claim that the extra light bouncing around the mirror box may result in a reduction in contrast, but that has been sort of hard to demonstrate.*

This might be most evident if you are using FF lens with poor lens coatings and without lens hood. Once you use lenses with modern lens coatings, optimized for digital sensors, and a good tight lens hood, this effect should be... difficult to detect.
That said, some people cover the viewfinder, because light can enter the camera from that side. It comes down to how close to perfection you want to get.

But yes, generally speaking, APSC cameras use the "sweet spot" of FF lenses. Some lenses have terrible corner sharpness and CA on FF, but this part of the image gets cut off on APSC camera anyway..

Honestly, the main problem is that few manufacturers really made true APSC lenses. Most APSC lenses were FF designs with slight tweaks. Many APSC lenses cover FF image circle (especially primes - there is a thread called DA lenses on FF that lists how supposedly-APSC DA lenses actually do on FF). So even if you look at your APSC lenses, they might already contain "FF lens weaknesses" (extra image circle, big size). Its not like the Q, where the lenses were made specifically for that image format and fit it perfectly, with image circle and no extra "wasted" glass.

All in all, not something to worry about, unless you are buying very specialized lenses where you pay a lot of extra money for the lens to cover FF image circle with edge-to-edge sharpness or if you are traveling and you need as compact a lens as possible. With modern lens coatings, lens hoods, and mirror box lining, the extra light is hardly a problem.

[tibbitts]

That said, some people cover the viewfinder, because light can enter the camera from that side.

This is a significant issue mostly because Pentax cameras (at least through the K5, I don't have a newer model) don't provide a built-in blind to cover the finder. Little strap-mounted covers are, I guess, better than nothing, but for those of us accustomed to the built-in blinds in other brands, even from decades ago, it's disappointing that our Pentax cameras don't have this feature.

[jeverettfine]

I am an old hand with large format 4x5 and 8x10 cameras and it is absolutely standard for the image circle to be significantly larger than the diagonal of the film. This allowed rise, fall, tilt and swing movements, critical to the full use and utility of the cameras. Using a lens with an image circle greatly larger than the film (sensor) diagonal is a complete non-issue, except perhaps for the most extreme pixel-peepers.
Also, light entering through the viewfinder should have no effect on the photo, though it could effect the exposure. Good lens shades that shield the lens from light sources beyond the image itself has always been a help with all cameras reducing flare etc. Modern optical coating of course have improved this as well making a shade less necessary. Whether it is a full frame lens or a crop lens should make no difference here. You just need a shade matching the relative angle of view.

[GUB]

Some of the old FF lens have a flat rear lens surface (Chinon 50mm 1.4 for example) and this is considered to give the potential of reflections of the highly reflective sensor surface. Don't know that is has been shown in practise though.

[osv]

and is there any disadvantages of using FF lens on crop bodies? I think this could help me making right choices for lens purchases. Thanks!

the only true comparisons are at an equivalent field of view, dof, distance, etc., so yes, you chop out edge weaknesses with a ff lens on crop, but what happens to edge quality with the equivalent lens on crop?

if 28mm ff shot at f/2 = 18mm crop shot at f/1.3 Digital Camera Sensor Sizes: How it Influences Your Photography

which lens is going to be the hardest and most expensive to design? forget the difficulty of 18mm at a fast f/1.3 aperture, keep it simple, which lens will generally have the least amount of distortion?

[clicksworth]

That is one way to look at it. The other is that the FF lens designer put most of his effort into making the FF edges work rather than concentrating on the crop portion that you will use. That being said, lens purchasing is always a correct choice.

[ScooterMaxi Jim]

Some of the old FF lens have a flat rear lens surface (Chinon 50mm 1.4 for example) and this is considered to give the potential of reflections of the highly reflective sensor surface. Don't know that is has been shown in practise though.

Absolutely, it has been shown. Anyone who converts a dSLR to infrared becomes well aware of the "hotspot" problem in the center of the image area. This essentially is the same issue, magnified based on the small infrared spectrum remaining, but having some impact on regular photography, as well.

This is an issue of film vs. digital design, not so much related to crop vs. FF image circle.

It isn't wise to generalize about how a FF film lens will perform on crop sensor digital. You are dealing with a lot of variables regarding not only the lenses, but changes in sensor technology. Very generally, newer, ultra-wide crop lenses tend to perform better in terms of freedom from CA and overall sharpness into the corners (if centered properly - which isn't as easy as it sounds). Full frame UWA lenses also have improved, but those are not as common (Samyang is the exception - with several fine, newer UWA FF designs.) The older film UWA lenses tend to fall short overall, but often can be corrected for the CA issues. Crop sensor lenses tend to smaller and lighter - for obvious reasons. Beyond that, it is hard to make broad statements.

[othar]

the only true comparisons are at an equivalent field of view, dof, distance, etc., so yes, you chop out edge weaknesses with a ff lens on crop, but what happens to edge quality with the equivalent lens on crop?

if 28mm ff shot at f/2 = 18mm crop shot at f/1.3 Digital Camera Sensor Sizes: How it Influences Your Photography

which lens is going to be the hardest and most expensive to design? forget the difficulty of 18mm at a fast f/1.3 aperture, keep it simple, which lens will generally have the least amount of distortion?

I think you went a little overboard here, when comparing lenses made for apsc and ff only on an apsc camera there is no need for any equivalence talk. A 50mm lens has the same FOV on a crop camera no matter if it was designed for apsc or ff.

As for the OP's question: I don't think there is a real disadvantage in using lenses designed for ff on a crop camera in general. The only thing to consider is that newer lenses usually are better corrected for CAs and have better lens coatings (on the other side the old lenses are often smaller because they use fewer glaselements), but that being said old lenses can still be great performers on digital cameras (my A 50mm f1.7 lens (old ff design) and my Samyang 35mm f1.4 lens (new ff design) are the sharpest lenses I own).
The only disadvantage I can think of, is that the designated lens hoods on ff lenses are too short for the use on apsc cameras.

[Na Horuk]

The biggest difference comes if you look at specialized zoom lenses made for either format, like comparing DA*50-135mm f2.8 and DFA 70-200mm f2.8. Both are good, but the FF lens is much bigger, heavier and more expensive.
Once you are dealing with primes, the differences are smaller. Bigger problem is using lenses from even larger formats, like 67 film era lenses adapted to APSC. FF and APSC are fairly close together to begin with.

[gofour3]

it's disappointing that our Pentax cameras don't have this feature.

Some old Pentax film bodies (35mm, 645 & 6x7) did have an eye-piece blind, either built in or using an accessory plastic cover.

The LX did not need one, as it metered off the film plane.

Phil.

[osv]

I think you went a little overboard here, when comparing lenses made for apsc and ff only on an apsc camera there is no need for any equivalence talk. A 50mm lens has the same FOV on a crop camera no matter if it was designed for apsc or ff.

no need for equivalence? gee, maybe you'd better email cambridge in colour and tell them that their calculator is all wrong.

As for the OP's question: I don't think there is a real disadvantage in using lenses designed for ff on a crop camera in general.

in a word, no: "Neither the focal length nor the f-ratio of a lens change as a function of sensor (for example, a 50mm f/1.4 lens is a 50mm f/1.4 lens, regardless of the sensor behind the lens). However, the effect of both the focal length and the f-ratio on the visual properties of the photo very much depend on the sensor, and scale in direct proportion to the size of the sensor.

25mm f/1.4 on mFT (4/3) is equivalent to 31mm f/1.8 on 1.6x (Canon APS-C), 33mm f/1.9 on 1.5x (APS-C for everyone else), and 50mm f/2.8 on FF (FX), where "equivalent to" means:"

Equivalence

[othar]

in a word, no: "Neither the focal length nor the f-ratio of a lens change as a function of sensor (for example, a 50mm f/1.4 lens is a 50mm f/1.4 lens, regardless of the sensor behind the lens). However, the effect of both the focal length and the f-ratio on the visual properties of the photo very much depend on the sensor, and scale in direct proportion to the size of the sensor.

25mm f/1.4 on mFT (4/3) is equivalent to 31mm f/1.8 on 1.6x (Canon APS-C), 33mm f/1.9 on 1.5x (APS-C for everyone else), and 50mm f/2.8 on FF (FX), where "equivalent to" means:"

Equivalence

I think you misunderstood my post. I am not saying the sensor size has no influence on the field of view you get with the same focal lenght, I was just saying that the OP just wanted to know about possible advantages or disadvantages of useing lenses designed for ff on a crop camera.