Rangefinder Producing More Bokeh?

[jcb4718]

Sevo has a point. Aperture is a kind of geometrically defined quantity related, but not precisely related, to the light hitting the film plane...and certainly not precisely enough for the motion picture business -hence their use of t-stops to get reproducible exposures from scene-to-scene. As I said above, I found a big difference in bokeh at a given f-stop between my 50mm f1.8 and f1.4 lenses. This is due to some of the off-axis the light being absorbed by the 'walls' of the f1.8 lens and therefoire not reaching the film plane. As I said, this produces out-of-focus highlights that are not circular: they are flattened and symmetrical about the centre of the image ('swirley bokeh'). It also means that for the f1.8 lens less light is reaching the film plane at a given aperture than with the f1.4 lens, particularly towards the edge of the image (= vignetting). It's a complex business, I guess that's why the 'Director of Photography' is so important on a movie set!

 

[peterm1]

No, they limit it. There are additional losses in the glass and coatings, not accounted for in f-stops, which are calculated from focal length and aperture diameter. If you want stops as a figure of light at the film plane, use speed in t-stops (which are measured and hence include all transparency factors) like the motion picture industry.

Understood and acknowledged but with the utmost respect, you are hair splitting. It makes limited difference in most situations so for practical purposes we all use F stops.

So given the same ambient lighting, within the limits you have described F8 should within workable limits, be the same no matter what camera or lens or format you are using.

When I hold up my old hand held meter, it tells me what the ambient lighting (as a measured EV value) and then allows me to use a sliding scale to convert this into a range of acceptable F stop and shutter time alternatives for the film sensitivity I have dialed in. Without a standardised value of this sort, it would be impossible to have light meters.

My statement stands but yes there are limits to the accuracy of f stops as a measure of light hitting the film plane but in practice it works.

 

[sevo]

Understood and acknowledged but with the utmost respect, you are hair splitting. It makes limited difference in most situations so for practical purposes we all use F stops.

Common differences between f and t stops are in the 0.2-0.5EV range. A third stop error is irrelevant in photography where we rarely light and meter to half stop precision - but it is critical in motion picture use, where material shot with different lenses on different days is mixed in the cutting. A third stop brightness difference between shot and reverse already results in visible brightness or contrast jumps (or more lab cost for optical copies than previously calculated, to hide that problem).

 

[BNLee]

jcb4718, I see... Maybe you are doing too much research on your cameras? Haha... Just kidding, really have respect for people who go all the way to analyse the way their camera works, you too Peter! Makes me feel like I need to go back to some physics classes...

Yes, I know the title is misleading, I mean the amount of depth of field in the rangefinder and an SLR, at a same focal length and same aperture.

Inverse square law... That brings me back... Thanks for the lengthy reply too Peter, I understand that the f64 on a medium format looks like a f11 on 35mm, as the film size is substantially bigger.

Thanks for the replies too everyone!