Originally posted by brandonbpm Can you explain a little better the relationship between ff and dof if shooting wide-open? It was a little confusing to me.
I am not an expert but I will try. Others are much more knowlegeable than I am.
Originally posted by Pål Jensen Theres no such thing as shallow DOF flexibility. FF has the DOF range moved one stop towards shallower DOF than APS (it looses in the other end). This is an advantage if want as shallow DOF you can get with a certain lens but a disadvantage if you don't (most of the time if you don't shoot wide open all the time). For the same DOF you have to use one stop higher ISO value on FF than on APS in order to achieve the same shutterspeed at a certain DOF.
Lets assume you had an f1.8 lens on a full frame and a f1.8 lens (same focal length and all things being equal) on a 1.5 crop camera... you will have different DOF for each at the exact same distance to the focal plane.
A Flexible Depth of Field Calculator
Use the calculator above to see...
According to it using a 50mm f1.8 at a 5 meter distance to the focal plane with a FF will give you 1.16meters of DOF...
Same exact calculation using a 1.5 crop camera...gives you a .76 meters DOF. According to that its actually thinner with the crop sensor using the exact same lens.
In a full frame format according to that calculator you need f1.2 to get a .77meter DOF at the same exact focusing distance mentioned above.
He's saying this same phenomenon continues on throughout the range regardless if you are focusing at 5 meters away, 10 meters, 20 meters or so forth...its the relationship of fstop # to actual DOF relationship...according to my calculation on that calculator a FF camera inherently has a 'fatter' DOF.
The calculator is just a rough guide though.
I am not sure if the calculator takes into account different lens designs... some lenses are actually built for crop sensor cameras. Everything is simply sized down to match the sensor so it alters the optical formula. With Pentax (and Canon I assume) you can use a lens designed for a FF camera on a crop sensor camera and this too would have an effect on 'actual effective DOF' and it might not be exactly what that calculator says it will be...it just depends on the combo.
In the second part of his post (if I read and understand it myself) was that--as I said above to get an equivalent DOF on a FF vs a crop you will have to use a different fstop #. He is in effect saying (again my words not his--so maybe I am wrong)...that if you start your calculation by saying 'I want this much DOF' and then work backwards you will have to use different fstops to obtain the same thing on the different formats and because its literally a different fstop you then have to adjust the ISO if the shot requires an equivalent shutter speed.
If you have leeway to go with slightly faster or slower shutter speeds you might have to either use a higher ISO on the camera or a slower shutter speed to get the same shot....
Again, I am just an innocent bystander here
I am also confessing up front that I am relatively new to photography (but I have read a lot)...again the others can feel free to correct me if I am wrong...