[disord3r]

this has nothing to do with focal length equivalence between sensors

I have a K-50. When Ricoh/Pentax announced the FF body, I was stoked. When I got wind of the pricing, I was super stoked. In anticipation of eventually buying a K1, I decided that I ought to focus any lens purchases on lenses that would be compatible with the FF body, whether designed for film or digital. I have read, however, that this may be a bad idea for crop bodies.

Lenses, whether FF or not, make the amount of light gathered available to an imaging circle that is designed to be more or less the same size as the sensor's diagonal. But the aperture values should be considered with respect to the sensor size it was designed for, no? If I'm using a FF lens on a crop body, of course I have to take the focal length equivalence into account so I know the angle of view I'm getting, but in addition to cropping out pixels that the crop sensor will see, I'm also cropping out light. I therefore should have to apply an equivalence to the aperture value as well, correct? A FF lens that is f/2.8 may be so on a FF body, but on a crop body, not all of that light is reaching the sensor.

I'm prepared to deal with this for the time being, but what does this mean for DA lenses like the 50 1.8 or 35 2.4 which are apparently designed for a FF-ish imaging circle? Does that mean that they aren't really functioning at their stated aperture value at all when mounted on a crop body? They gather the same amount of external no matter the body they are attached to, but whether the sensor is able to use that light is a different story. The aperture value will be read by a crop body as 1.8 or 2.4 respectively, but how does the body deal with that? Does it not care, and just meter the light off the sensor? What is is even using those values for if not for some sort of calculation?

[vonBaloney]

this has nothing to do with focal length equivalence between sensors

I have a K-50. When Ricoh/Pentax announced the FF body, I was stoked. When I got wind of the pricing, I was super stoked. In anticipation of eventually buying a K1, I decided that I ought to focus any lens purchases on lenses that would be compatible with the FF body, whether designed for film or digital. I have read, however, that this may be a bad idea for crop bodies.

Lenses, whether FF or not, make the amount of light gathered available to an imaging circle that is designed to be more or less the same size as the sensor's diagonal. But the aperture values should be considered with respect to the sensor size it was designed for, no? If I'm using a FF lens on a crop body, of course I have to take the focal length equivalence into account so I know the angle of view I'm getting, but in addition to cropping out pixels that the crop sensor will see, I'm also cropping out light. I therefore should have to apply an equivalence to the aperture value as well, correct? A FF lens that is f/2.8 may be so on a FF body, but on a crop body, not all of that light is reaching the sensor.

I'm prepared to deal with this for the time being, but what does this mean for DA lenses like the 50 1.8 or 35 2.4 which are apparently designed for a FF-ish imaging circle? Does that mean that they aren't really functioning at their stated aperture value at all when mounted on a crop body? They gather the same amount of external no matter the body they are attached to, but whether the sensor is able to use that light is a different story. The aperture value will be read by a crop body as 1.8 or 2.4 respectively, but how does the body deal with that? Does it not care, and just meter the light off the sensor? What is is even using those values for if not for some sort of calculation?

No. Just as 50mm is 50mm is 50mm, so is f2.8 is f2.8 is f2.8. It is a physical property of the lens. As far as not "using all the light", the smaller sensor is still getting the exact same amount of light that THAT SAME PORTION OF A LARGER SENSOR would be getting. Bottom-line, there is no issue with using FF lenses on crop sensors. (Well, not these issues anyway.) Using a crop body is EXACTLY the same as-if you had used a FF body (with a larger version of the same sensor with same pixel density in the crop portion) and cropped it, chopped the edges off. That's why they get called that.

[Kerrowdown]

are FF lenses on crop bodies a bad idea?

I'd have been a bit stuck with my "Ladies" for the last eight years or so if that was the case.

[RKKS08]

You are right only in the sense that the total amount of light used is only a part of what meets the image circle.

But for the exposure this is irrelevant, as here decides the amount of light per area. And this stays the same, no matter how big the sensor is, or how much you will crop.

[TwoUptons]

There's a whole other world of aperture equivalence that I'm not going to touch...

Functionally, for most people, the only difference is going to be lens size. A Pentax 105mm lens for a 67 is going to be bigger than one for K-mount. But mount them both on my K-01 (I'm willing to demonstrate if someone will send me both lenses and a 67 adapter), set them both to, say, f4, and you'll get the same result.

Sure the 67 lens has 'wasted' a bunch of light, but barring weird internal reflections, the image doesn't care...

-Eric

[UncleVanya]

The only downside is weight. The full frame and crop lenses of the same focal length and f stop are treated the same on the same sensor.

[paulh]

You are overthinking the "problem". At least 75% of my lenses are FF, and I've been using them on crop for years. Just use them and enjoy

[MadMathMind]

There's a whole other world of aperture equivalence that I'm not going to touch...

I really dislike that argument. It's incredibly confusing and based on "how it will look" but is overly abstract. It's trying to be a reverse of the focal length equivalent, which is bad enough.

[UncleVanya]

It is certainly true that you need a wider aperture to get the same depth of field as sensor sizes go down. The exact amount is debatable.

As I understand this, it is largely due to the fact that lenses of the same focal length are used for different purposes on different sensors. A 15mm on a q may have the same depth of field at the same distance as on a k-3, but the view will be totally different and if compared with the "equal" lens it will look as though the smaller sensor had increased depth of field.

[Bruce Clark]

this has nothing to do with focal length equivalence between sensors

If I'm using a FF lens on a crop body, of course I have to take the focal length equivalence into account so I know the angle of view I'm getting, ...............

Not me. I use the viewfinder to evaluate my angle of view. Gasp! I don't even take a calculator with me to even calculate focal length equivalence let alone a protractor to calculate angle of view. For the same reason I don't buy a new car every time I go shopping or on holiday because the load or terrain will be different. I do not need to use 100% of capacity or capability at 100% efficiency, 100% percent of the time. What I decide to crop a photo in PP? By following the equivalence argument, does this act somehow invalidate my original exposure?

In my humble opinion whoever invented the term equivalence has a lot to answer for.

[clackers]

this has nothing to do with focal length equivalence between sensors

I have a K-50. When Ricoh/Pentax announced the FF body, I was stoked. When I got wind of the pricing, I was super stoked. In anticipation of eventually buying a K1, I decided that I ought to focus any lens purchases on lenses that would be compatible with the FF body, whether designed for film or digital. I have read, however, that this may be a bad idea for crop bodies.

Lenses, whether FF or not, make the amount of light gathered available to an imaging circle that is designed to be more or less the same size as the sensor's diagonal. But the aperture values should be considered with respect to the sensor size it was designed for, no? If I'm using a FF lens on a crop body, of course I have to take the focal length equivalence into account so I know the angle of view I'm getting, but in addition to cropping out pixels that the crop sensor will see, I'm also cropping out light. I therefore should have to apply an equivalence to the aperture value as well, correct? A FF lens that is f/2.8 may be so on a FF body, but on a crop body, not all of that light is reaching the sensor.

I'm prepared to deal with this for the time being, but what does this mean for DA lenses like the 50 1.8 or 35 2.4 which are apparently designed for a FF-ish imaging circle? Does that mean that they aren't really functioning at their stated aperture value at all when mounted on a crop body? They gather the same amount of external no matter the body they are attached to, but whether the sensor is able to use that light is a different story. The aperture value will be read by a crop body as 1.8 or 2.4 respectively, but how does the body deal with that? Does it not care, and just meter the light off the sensor? What is is even using those values for if not for some sort of calculation?

Where did you read this, disor3r?


It wasn't by the bulk of people on this forum, who know you ward off Total Light and Equivalence nutters by wearing garlic and brandishing a crucifix.


A pixel on a FF or cropped sensor gets the same amount of light from the DA50 at f1.8.


I've shot on FF using both your plastic fantastic examples, and the exposure for a scene is exactly the same as when I shoot using my K-30.

[Digitalis]

A FF lens that is f/2.8 may be so on a FF body, but on a crop body, not all of that light is reaching the sensor.

Wrong, Assuming the flange is kept the same the intensity of light at the focal plane will be the same regardless of the format the light is projected on*. Think of it this way: if you break a 36mmX24mm in half, will the smaller pieces need more light to see your reflection in them?


* providing the format is within the limits of what the lens is designed for.

[MadMathMind]

It is certainly true that you need a wider aperture to get the same depth of field as sensor sizes go down. The exact amount is debatable.

As I understand this, it is largely due to the fact that lenses of the same focal length are used for different purposes on different sensors. A 15mm on a q may have the same depth of field at the same distance as on a k-3, but the view will be totally different and if compared with the "equal" lens it will look as though the smaller sensor had increased depth of field.

This is correct. People put as much of their subject in the frame as possible. Let's use the FA31 and FA43 for comparison, putting the 31 on a crop body and the 43 on a full-frame. The FA31 has an equivalent FoV on APS-C as a 46mm lens on a full-frame camera. For the most part, the FA31 on APS-C and FA43 on FF will fill the frame in the same way given a certain distance-to-subject.

Depth of field and strength of background blur is dependent on a number of things:
1) Focal length
2) Distance to subject
3) Distance from subject to background
4) Aperture

The relevant values here are #1 and #2. #2 is constant because of the nearly equivalent FoVs here. But the FA43 is focused closer for its focal length than the FA31 is. As a result, the FA43 FF shot will have more and deeper blur because a longer focal length lens is used on it. This effect becomes more dramatic as you move up: an FA50 on crop and FA77 on FF will have nearly the same composition from a given vantage point but the FA77 will have much stronger blur because the focal length is much longer for the focus distance.

On crop, the FA77 doesn't produce the strength of blur it would on FF because you have to back up another 5 feet or so to get everything you want in the frame.

[disord3r]

You are overthinking the "problem".

Nope. I just have a fundamental misunderstanding of how these things work. They are filled with magic and/or black holes.