Aperature translation question ... confused !

Hi All,

I have just finished reading Bryan Peterson's book "Understanding Exposure" as it came so well recommended on this site.

I have to say it is an excellent book and has made a lot of concepts much clearer to me.

There was something though that I did not fully understand. At some point in the book Bryan says something along the lines of the aperature on the lenses of digital SLRs do not match those of traditional 35mm SLRs. e.g. f/4.5 on my lens would actually equate to something ridiculous like f/16 ... ish LOL.

So does this mean that it is hard / impossible for me to take pictures where the subject is in focus, but the background is blurred ( bokeh ) or do I need to invest in a lens which has a huge aperature rating in order to product this type of shot ?

In his book Bryan often uses aperatures of the order f/2.8 in order to take this type of shot.

Sorry if this is a ridiculously confusing question, but I don't have the book with me right now.

Many Thanks,
D

What he means is that all else being equal (focal length, focal distance, aperture) a smaller sensor or piece of film will show more DOF than a larger one, or conversely a bigger sensor will show more bokeh at a given aperture, hence why large format cameras are often shot at f/32 and even smaller to get adequate DOF. In a modern DSLR the effect is not too pronounced, since the sensors are approaching the size of a piece of 35mm film which is, I suppose, the "benchmark" for DOF. A 1.6x crop sensor camera will probably get the same subject separation at f/2.8 as a FF sensor will at f/4 (this is a guess, but probably in the right ballpark). With compact cameras with their very small sensors the effect is very pronounced and it would be difficult to separate the subject properly at any aperture.

Incidentally, a better way to think about this is that with a smaller sensor you get further away from the subject (using the same focal length) or use a shorter focal length to achieve the same field of view - as you get further away or use a shorter lens, the DOF increases. Conversely with a bigger sensor you have to move in closer (or use a longer lens) to maintain the same field of view so DOF decreases.

The aperture is the same, f2.8 is the same irrespective of what camera its on.

The difference is in the apparent field of view. A DSLR with an APC-S sensor(or crop format) has a smaller field of view ( as the image is focused onto a smaller area) than a full frame. This has the effect of increasing the depth of field.

The identical shot taken with a crop at f2.8 will look like a f4 on a 5D.

The wider the aperture the more blurred the background basically though.

Thanks for help guys. I have a 400D with the kit lens so don't have a sparkling wide aperature to play with .

Time to save up for a bigger aperature lens I think

Thanks
D

Yeah but you've got LiveView with all the superimposed goodie info, and there you can learn a mountain through monkey see - monkey do.

sanderd wrote in post #5927882
Time to save up for a bigger aperature lens I think

Wouldn't discourage you from saving up for a bigger aperture lens at all, but at the same time it is rare that you will want much more separation than that lens will give you. Perhaps a stop more might be very useful, but much more of a wider aperture than that won't, IMHO, get used as much as you think after the initial novelty has worn off - razor thin DOF can look great, but is not the easiest thing to consistently pull of well, since it can easily be too shallow.

To get the shallowest DOF possible from your kit lens use the longest focal length you've got, and get the subject up as close as you can (and obviously use your widest aperture) - both lowering focal distance and raising focal length decrease DOF.

PaulBradley wrote in post #5929253
To get the shallowest DOF possible from your kit lens use the longest focal length you've got, and get the subject up as close as you can (and obviously use your widest aperture) - both lowering focal distance and raising focal length decrease DOF.

Thanks for the tips. I will try those techniques next time.

D.