[stevebrot]

Really tho, the point of my entire thread was not to be mathematically accurate about the comparison between crop and full frame, it was more for me to get an idea of what I am seeing when swapping lens around between crop and ff bodies, what the hypothetical equivalent would be like. Not exact... like.

The short answer is to not sweat it. Consider that the viewfinder on the K-1 offers 0.70x magnification while that on the KP and all other recent APS-C models offer 0.95x magnification. The apparent DOF in one is similar to that in the other. Neither are completely accurate* for all focal lengths but the difference in magnification essentially corrects for the relative difference in DOF. If you are confident with working with the K-1 view of things, your Pentax APS-C dSLR should provide a similar view.


Steve

* Even with the nominal reference of 50mm lens at infinity, the viewfinders for both formats show exaggerated DOF compared to an 8x10 print or the size many of us work with on our computer monitors.

[stevebrot]

Bruce, I believe we are saying the same thing in confusing ways. The reason people hate equivalence is because for any help is could be, it will cause 10 times the confusion.

It also has zero practical application when moving from one format camera to another. Any attempt at such results in "overthinking" the process of making photos. If I am particularly concerned about DOF, then I should shoot as if it is a concern, regardless of format.


Steve

[Rondec]

It also has zero practical application when moving from one format camera to another. Any attempt at such results in "overthinking" the process of making photos. If I am particularly concerned about DOF, then I should shoot as if it is a concern, regardless of format.


Steve

I don't think it has zero application. If I am interested in portraiture and I am shooting with an FA 77 or Sigma 85mm f1.4 on a K-1 and I usually shoot at f2 or f2.8 with those lenses, will I be able to get similar results on a micro four thirds camera and if so, what lens do I need to get those results? Will that lens be smaller than the FA 77?

I don't do it as much now, but when I first bought a K-1, I was very much used to how focal lengths worked on my K3. Equivalence did help me understand how to emulate some of my favorite focal lengths from APS-C on full frame. I understand that it has zero application to you, but it did to me when I was first learning how to use 35mm.

People who have shot lots of different formats and are experienced don't need it, but there are plenty of beginners who can benefit from it and I don't think it has to be confusing or over thinking. It is as simple as saying "I really liked the DA 15 limited on my K3 at f8, I guess I need something around 20mm and shoot it around f11 to get similar results on my K1."

[swanlefitte]

It is as simple as saying "I really liked the DA 15 limited on my K3 at f8, I guess I need something around 20mm and shoot it around f11 to get similar results on my K1."

A cool alternative is to learn a standard lens well. For fullframe a 50 is great because of the round number. Then you know what you frame with the 50mm at 5m with 1.7m dof, is what you can frame with the 100mm at 10m with 1.7m dof, is what you can frame at 15m with the 150mm with 1.7m dof, etc.
You already know what the lens for the other system is as soon as you use the standard lens once.

[Lowell Goudge]

It also has zero practical application when moving from one format camera to another. Any attempt at such results in "overthinking" the process of making photos. If I am particularly concerned about DOF, then I should shoot as if it is a concern, regardless of format.


Steve

I agree, the only thing worse than an imperfect shot, is..........

No shot at all because you spend too much time thinking

[bdery]

I know the aperture is still showing 1.8, but is it performing the same in terms of DoF that a 115mm/1.8 lens would on the K-1?

AGain, trick question. You essentially want to create the same image on two different sensors. Your way of thinking about it is an approximation, which will work in general, but will not be precise. There are too many variables involved, I think, to make of the cufff comparisons while shooting.

ou see... I don't really think of lenses in terms of FL, I think in terms of FoV. I have to, because I shoot lenses on two different sensor bodies. A 28mm on a K-1 is pretty much the beginning of a wide angle shot (group portrait), the same lens on the KP and it's a straight up portrait lens. I've come to learn and think of FF as being 'the scale' for me to reference things, so that a F28 on a KP and a FA43 on a K-1 is kinda duplicating a similar shooting experience, and is not covering a big difference in FoV range. And thus far in my journey I have only been considering FoV differences, but not aperture

Wouldn't it be easier to just learn how each lens behaves on each body? I use the K-1 mainly, but also the K-3. When mounting a lens, I more or less know what to expect.

I understand that you reason in FOV, but then when you toss in aperture it gets more complicated. Again, the FOV part is easy, one is a crop of the other, but once you wish to achieve the same image with two systems,..

[Rondec]

A cool alternative is to learn a standard lens well. For fullframe a 50 is great because of the round number. Then you know what you frame with the 50mm at 5m with 1.7m dof, is what you can frame with the 100mm at 10m with 1.7m dof, is what you can frame at 15m with the 150mm with 1.7m dof, etc.
You already know what the lens for the other system is as soon as you use the standard lens once.

How is that easier than saying I like a 15 mm lens at f8 so I would like a 20-ish mm lens at f11 on a different size sensor? To me it is more confusing. This is particularly true since all of the camera brands list "35mm equivalent focal lengths" with their lenses.

I guess the thing to me is that if aperture is the same regardless of sensor size then why shouldn't everyone buy into micro four thirds, where you can get a 12-40 f2.8 lens that only costs 800 dollars and weighs 382 grams (compared with a lens like the Pentax DFA 24-70 f2.8 that doesn't even go to 80mm, costs a thousand dollars and weighs 787 grams). f2.8 is f2.8 with regard to exposure and yet it gives very different results depending on the sensor size. Equivalence merely tries to put that into numbers.

The best thing is to do no conversions. To do what bdery says and just shoot with the cameras you have until you figure out what works for you. I have no problem with that, but I have major issues with people converting only the focal the length and ignoring the aperture because "f2.8 is f2.8."

[swanlefitte]

How is that easier than saying I like a 15 mm lens at f8 so I would like a 20-ish mm lens at f11 on a different size sensor? To me it is more confusing.

I never said easy. I said interesting alternative. You have to learn one lens well If you don't have that, it's confusing.
If you see 3 people 50 yards away what equivalence you gonna use to figure it out? Put lenses on one camera until you get the right one. Then compute equivalence and use that one? For this just put lenses on the other camera and forget equivalence.
Or a neat alternative.
If I am in the field and I see 3 people at 50 yards I know that the 50 needs close to 20 yards to frame 3 people so a 2.5x lens.
How you gonna use the equivalence for that? Now I change to apsc and I know the 35x2.5x or the 85-90mm will allow me to frame the shot. I am not using exact numbers like 43mm so it's not perfect. I just use easy approximations, 50, 35.
It can be confusing. So is using manual mode without some training. Manual mode is a neat alternative to auto mode that allows even more control same as this way.
Convert a 70mm apsc to m43? It is easy. Normal lenses are 35 and 25. So a 2x or 50. Simple.

[fs999]

You shouldn't put a FF lens on a crop sensor !
...or is it the other way ?

[stevebrot]

It strikes me as strange that nobody asks this sort of question when adapting 6x7 lenses to 24x36mm FF.

On my shelf are ILC cameras capable of shooting APS-C, 24x36mm, 6x6cm, 6x7cm, and 4x5". Other than keeping straight the conventions for "wide", "normal", "portrait", and "long" for each format, I don't really have any concerns regarding replicating a "look" between cameras. In short, it ain't going to happen and the rules are "horses for courses". Comments for each format run like this...

• APS-C: Love my K-3, but still find it hard to find quality wide angles at moderate price in the 15mm to 20mm range. F/1.4 wide angles? (rolls eyes) It is hard enough finding focus at f/2.8. For macro and extra reach, APS-C is so very nice, particularly since cheap and ubiquitous fast 50s work so well for portraiture with this format.
• 24x36mm: Sweet and familiar after over five decades shooting in this space. Fortunately what I learned there is applicable to APS-C and vice versa.
• 6x6 and 6x7: True medium format film was a revelation.* Yes, there is potential for amazing quality, but DOF is dear (there is often very little to work with) and lenses are slowish. Amazing how ISO 400 is the new ISO 100.
• 4x5: Everything from medium format plus the note that large format cameras have movements for a reason. DOF is so limiting that many subjects are inaccessible without manipulating the position of lens to film. Fine focus is done using a magnifier loupe on the ground glass and DOF is estimated based on experience. Macro 1:1 is usually easy to accomplish, but with a 6" frame diagonal, subjects are usually not very small.

Now if someone wants to dial in 2/3 stop narrower aperture moving from 24x36mm to APS-C while bumping the ISO down to allow the same shutter speed and equivalent noise to a larger sensor, that is fine. If doing so fits one's flow, it is fine with me.


Steve

* I dabbled with 645 film at one point and found the negative to not offer a whole lot more than 35mm film.

[bdery]

...seems to me there's less light hitting the crop sensor, but that's only because the light at the edges of the projection exceed the dimensions of the sensor - but it's just as bright at the center as it would be in either camera.

Yes, there is less TOTAL light reaching the sensor. There is the exact same amount of lens per surface area.

The second part of your sentence is exact.

[mikesbike]

You shouldn't put a FF lens on a crop sensor !
...or is it the other way ?

The issue you are bringing up is the physical width of the lens elements being able to afford coverage to the sensor, according to its size. From knowing that you can easily see it is the other way. A crop lens is not as physically wide, just like the crop sensor. So it would not likely offer adequate coverage for the larger FF sensor- a situation that would be susceptible to producing vignetting.

[angerdan]

It seems this discussion get started every year - although phyiscs are the same since the invention/discovery of photography.

Some examples:
Lens Aperture and Sensor Size - PentaxForums.com
On aperture equivalence: are FF lenses on crop bodies a bad idea? - PentaxForums.com
f-stop crop - PentaxForums.com
dumb f-stop question - all f-stops created equal? - PentaxForums.com
caclulating f-stop relationships - PentaxForums.com
DOF and F-stop question. - PentaxForums.com

More clarification:
T-stop vs. F-stop In a Camera Lens - Articles and Tips | PentaxForums.com
Understanding Crop Factor | B&H Explora
Depth of Field, Part III: The Myths | B&H Explora
What is equivalence and why should I care?: Digital Photography Review
This Crop Factor Calculator Makes Sensor Math a Breeze
Aperture - Wikipedia
A Tedious Explanation of the f/stop

Bringing in the correct names:
aperture (written, can be f/t-stop, open, close or relative to the minimum/maximum of an lens)
t-stop (for transmission stops, by convention written with capital letter T) is an f-number adjusted to account for light transmission efficiency (transmittance)
f-stop (mathematical result of dividing focal length with lens diameter)
equivalent f-stop (regarding depth of field, to get the same sharpness on another sensor)
lens speed (independent from sensor, like f/t-sop)

Conclusion:

the aperture number is a mathematical relationship between the focal length and the usable diameter of the object lens, the front/1st element.
Specifically the focal length divided by the usable diameter of the lens.

[fs999]

The issue you are bringing up is the physical width of the lens elements being able to afford coverage to the sensor, according to its size. From knowing that you can easily see it is the other way. A crop lens is not as physically wide, just like the crop sensor. So it would not likely offer adequate coverage for the larger FF sensor- a situation that would be susceptible to producing vignetting.

[mikesbike]

Good joke!