[BruceBanner]

Ok... So I get that when we have a FA 77/1.8 on a K-1 we are seeing a FoV that is 77mm and we are getting f1.8 for aperture. When we place the FA77 on a KP we are now getting a FoV equivalent to using 115.5mm lens (if such a lens existed) on the K-1, but do we also now get an aperture that is more like f2.7 along with it?

Or another way to look at it, if we had a lens that was intended for FF and sat on the K-1 and was 115.5mm f2.7 lens (just pretending here), then if we had a KP right next to the K-1 with a FA77, would they be identical in terms of FoV and aperture/exposure performance (just megapixel difference)?

Or another way... if we had a crop sensor lens that was 51.3/1.2, would that be the same as a K-1 with a FA77/1.8 on it?

If that is the case... then if we have a lens intended for Crop, such as a DA 12-24, and we put it on FF, then does the aperture appear wider than f4 (like it's doing the same as above but in reverse lol).

Hehe...

TIA!

BB

[bertwert]

Exposure wise the aperture does not change.
Using the 77 Ltd on a K-3 and on a K-1 at ISO 100, 1/100s and f/1.8 both cameras will produce images exposed identically.

I don't like this whole equivalency thing.
A 77/1.8 used on full frame is still a 77/1.8 when used on APS-C.
On a crop sensor, the image is just cropped. It's the same image as from a full frame sensor, but you only see a smaller bit of it.

[luftfluss]

Ok... So I get that when we have a FA 77/1.8 on a K-1 we are seeing a FoV that is 77mm and we are getting f1.8 for aperture. When we place the FA77 on a KP we are now getting a FoV equivalent to using 115.5mm lens (if such a lens existed) on the K-1, but do we also now get an aperture that is more like f2.7 along with it?

Or another way to look at it, if we had a lens that was intended for FF and sat on the K-1 and was 115.5mm f2.7 lens (just pretending here), then if we had a KP right next to the K-1 with a FA77, would they be identical in terms of FoV and aperture/exposure performance (just megapixel difference)?

Or another way... if we had a crop sensor lens that was 51.3/1.2, would that be the same as a K-1 with a FA77/1.8 on it?

If that is the case... then if we have a lens intended for Crop, such as a DA 12-24, and we put it on FF, then does the aperture appear wider than f4 (like it's doing the same as above but in reverse lol).

Hehe...

TIA!

BB

You might find this site interesting for playing with how various focal lengths and apertures are affected by the site of the sensor: Online Depth of Field Calculator

[pschlute]

would they be identical in terms of FoV and aperture/exposure performance (just megapixel difference)?

Bruce.....you must have a hand held light meter somewhere in your bag. Have a look at it and tell me where you input the sensor size into the calculation

---------- Post added 12-13-19 at 05:23 AM ----------

You might find this site interesting for playing with how various focal lengths and apertures are affected by the site of the sensor

It is a good site , but focal lengths are not affected at all whatever size sensor you mount them on.

[disconnekt]

When using a FF lens on a crop body, the only thing that changes is the field of view, everything else (focal length, f-stop) stays the same

[BruceBanner]

Bruce.....you must have a hand held light meter somewhere in your bag. Have a look at it and tell me where you input the sensor size into the calculation

---------- Post added 12-13-19 at 05:23 AM ----------



It is a good site , but focal lengths are not affected at all whatever size sensor you mount them on.

Nop... I've been winging it this far...

[swanlefitte]

You have a k1. Use it in crop and regular mode and compare. The difference is when you frame the same you enlarge the crop 1.5x more to be the same size. This makes those fuzzy dots that are too small to look fuzzy (circle of confusion) grow big enough where the fuzzier ones don't look like dots anymore thus meaning the dof is 1.5x less.

[stevebrot]

These things we know...

• Focal length is focal length. One can measure it on an optical bench, no camera needed.
• Aperture diameter is, well, the aperture diameter. One can measure it with a ruler and again, no camera needed.*
• F-number (relative aperture) is a fraction of the first over the second. That is why it is always expressed as a fraction. Again, no camera needed.

There are those who attempt to complicate things using concepts such as total light or various depths of equivalence, but from the perspective of optical physics, the three points above stand.

Steve


* Technically, not quite. It is the diameter of the entrance pupil; which may be and often is, different than the physical aperture diameter.

[swanlefitte]

Edit. Pretty sure thispart in ** is wrong. Maybe someone can explain, below the ** should be correct. Thought my calculator was based on 7x enlargement not enlargement to 8x10.
*The math for my comment based off depth of field calculator for a specific.
Apsc 40mm 2.8 10 meters Dof=7.54m
FF 60mm 2.8 10 meters Dof=4.96m
Enlarge the apsc 1.5x enlarge fuzzy dots so 7.54/1.5=5.02m Dof

Or decrease FF by 1.5 so 4.96 (1.5)=7.44
Or drop the FF aperture to 4 and Dof=7.45*

INSTEAD
At 10m f2.8 40mm the k1 in crop or full records the exact same in the part of the sensor it uses. But dof is shallower in crop. Dof crop=7.54m, dof full=14.66m. In other words the exact same pixels enlarged more look fuzzier.

[From1980]

Exposure wise the aperture does not change.
Using the 77 Ltd on a K-3 and on a K-1 at ISO 100, 1/100s and f/1.8 both cameras will produce images exposed identically.

I don't like this whole equivalency thing.
A 77/1.8 used on full frame is still a 77/1.8 when used on APS-C.
On a crop sensor, the image is just cropped. It's the same image as from a full frame sensor, but you only see a smaller bit of it.

Right On.
This has been discussed before so we should clarify it right away or it threatens to become long and drawn out again. Similar to what bertwert said, the 77mm focal length is a measured physical property of that optical design and doesn't change regardless of whatever camera it's mounted on. F/1.8 is a calculated ratio from that focal length divided by the actual measured maximum diameter of the iris diaphragm. No amount of cropping can change the mathematics the exposure is based on.
The intensity of light striking any spot on a focal plane (film or sensor) within a given lens circle of illumination remains constant except for limitations in certain designs that cause fall off toward the edges. This ratio of the relationship of focal length to interior diameter results in a constant f number series so exposure values are the same lens to lens and format to format.

---------- Post added Dec 13th, 2019 at 01:28 ----------

* Technically, not quite. It is the diameter of the entrance pupil; which may be and often is, different than the physical aperture diameter.

Good point, I see stevebrot clarified this more in the time in took me to get back to my desk.

[othar]

but focal lengths are not affected at all whatever size sensor you mount them on.

Correct, unfortunately field of view is, that's why this discussion won't go away anytime soon.

Or another way to look at it, if we had a lens that was intended for FF and sat on the K-1 and was 115.5mm f2.7 lens (just pretending here), then if we had a KP right next to the K-1 with a FA77, would they be identical in terms of FoV and aperture/exposure performance (just megapixel difference)?

Not quite. Field of view and aperture regarding depth of field would go in the equivalence calculation (aperture with crop-factor squared I think) but your exposure is not influenced by sensor size. That said the whole equivalence thing doesn't have a real-life value in my opinion, apart from confusing people and getting more posts on the forum

[StiffLegged]

Do we calculate aperture when using FF lenses on crop bodies?

Lemme see...

No.

(there now, that was easy)

[bdery]

When we place the FA77 on a KP we are now getting a FoV equivalent to using 115.5mm lens (if such a lens existed) on the K-1, but do we also now get an aperture that is more like f2.7 along with it?

The "crop factor" is only a crop. Nothing else.

See it this way. If you take a picture with your K-1, then crop it after the fact via software, you don't change the pixels which remain. You have the exact same image, only with pixels removed around the borders.

An APS-C sensor deos just that. Nothing more. It throws away part of the image, without changing what's left.

When we place the FA77 on a KP we are now getting a FoV equivalent to using 115.5mm lens

That's a dangerous way to see the crop factor equivalent, because of the kind of complications you are not writing about.

Sure, the FOV is equivalent, but nothing else is. Assuming so leads to mistakes.

[luftfluss]

][/COLOR]It is a good site , but focal lengths are not affected at all whatever size sensor you mount them on.

Yes, you're correct, but I meant "focal lengths" in the sense that you can change focal lengths in the equation as well as distance-to-subject and aperture and sensor size and see how they all play together.

[mlt]

The other way to answer this is: when you put a crop sensor lens on a ff body does it automagically become a larger aperture, no it stays the same as does the physical focal length of the lens. Field of coverage and by association, depth of focus are the only changes.