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#1
11-08-2012, 05:53 PM
 theodog (Jeff) Every photon is sacred ! Join Date: Oct 2007 Location: Coonabarabran Posts: 1,037
F-Ratio's and APS-C Imagers

Hi All,

My question concerns the effective f.number for the APS-C chips. I know that the effective focal length of a lens will change (usually by 1.5 or 6). So a 10mm on full (35mm) frame is effectively 15mm on a APS-C, but how does this effect the f ratio? And is there a simple way of calculating this?
#2
11-08-2012, 08:22 PM
 RickS (Rick) PI cult recruiter Join Date: Apr 2010 Location: Brisbane Posts: 10,584
Quote:
 Originally Posted by theodog Hi All, My question concerns the effective f.number for the APS-C chips. I know that the effective focal length of a lens will change (usually by 1.5 or 6). So a 10mm on full (35mm) frame is effectively 15mm on a APS-C, but how does this effect the f ratio? And is there a simple way of calculating this?
Jeff,

In astro imaging we don't tend to think in terms of effective focal length. What we care about is field of view and image scale.

FOV (in minutes of arc) is approximately equal to (3439 x d)/F where "d" is the width or height of the sensor and F is the focal length. Both d and F should be in the same units, e.g. millimetres.

There isn't a standard APS-C sensor size which is why the crop factor varies. It is 1.52x for Nikon, 1.6 for Canon, etc. Let's take the Nikon APS-C sensor size which is 23.6 x 15.8mm. If we plug that into the calculation above and assume we have a 530mm focal length (like my FSQ-106ED scope) then we get a FOV of approximately 153x102 arc mins.

If we do the same calculation for a full frame sensor of 36x24mm then we get a FOV of approximately 234x156 arc mins. You'll notice that the FOV varies by the crop factor of 1.52 from the FOV for the APS-C sensor, so maybe this is still a useful idea.

So, FOV tells you how large a piece of the sky is going to be captured by your sensor on a scope with a specific focal length. Image scale tells you about the resolution of the image you might capture.

The angular size of a pixel in arc seconds is approximately (206265 x p)/F where p is the size of a pixel and F is the focal length. Once again, both must be in the same units, e.g. mm.

If have 9 micron pixels (0.009mm) then this calculation gives us an image scale of approximately 3.5 arc seconds/pixel for a 530mm focal length scope. That's the size of the little piece of the sky captured by each individual pixel.

If you're not excited by doing these calculations yourself, here's a Windows application that will do them for you:
http://www.newastro.com/book_new/camera_app.php/
It also has samples of some well known astronomical objects and can show you what they look like with different scope and sensor combinations.

You also asked what effect sensor size has on f ratio and the answer is that it has no effect at all. The f ratio depends only on the aperture and focal length of the scope. It is not related to the sensor size.

Hope that helps...

Cheers,
Rick.
#3
11-08-2012, 08:22 PM
 troypiggo (Troy) Bust Duster Join Date: Nov 2008 Location: Brisbane, Australia Posts: 4,833
The focal length doesn't change. A 10mm focal length lens is always a 10mm focal length lens. It's just that on a crop sensor, ie smaller than a full frame sensor, doesn't collect the same area of light, so the apparent field of view is narrower by whatever fraction of a full frame sensor it is (1.6x for APS-C). But it's still 10mm focal length.

http://www.bobatkins.com/photography...nd_lenses.html

F-ratio is not affected.

Depth of field is subtly affected, because it's a function primarily of f-ratio and distance to subject, but also circle of confusion, sensors etc do come into it.
#4
12-08-2012, 06:36 AM
 theodog (Jeff) Every photon is sacred ! Join Date: Oct 2007 Location: Coonabarabran Posts: 1,037
Thanks.

I had hardly begun to read when I had that 'Doh' moment.

Thanks for taking the time on my question.
#5
12-08-2012, 08:12 AM
 bojan amateur Join Date: Jul 2006 Location: Mt Waverley, VIC Posts: 6,016
There is no "effective" f-number, and no "effective" FL - those terms are wrong and highly misleading, and this tread is the proof of that.

"Effecive FL" was "invented" to help people wrap their minds around the lens focal length and their field of view when used with standard Leica format film (24x36mm) and/or APS format (16x22mm).
It indicates what FL on APS sensor (or film) would be needed on Leica format to have the same viewing angle.
A little better and more accurate term is "Crop factor" (1.6 in APS size case, but it involves mental arithmetic and it seems people do not like that..)... it tells us how much smaller is the field of view for a given FL and sensor size, compared to standard Leica format (24x36mm)
#6
12-08-2012, 09:04 AM
 Merlin66 (Ken) Spectroscopy Wizard Join Date: Oct 2005 Location: St Leonards, Vic Posts: 7,188
I think as we move further and further away from the "original" 35mm SLR, the concept of crop factor etc becomes meaningless.
I'm sure there are younger members who haven't even seen an ol' 35mm SLR.
#7
12-08-2012, 11:07 AM
 troypiggo (Troy) Bust Duster Join Date: Nov 2008 Location: Brisbane, Australia Posts: 4,833
It's not necessarily 35mm film, it's full frame sensor. Just so happens that full frame sensors are similar size to 35mm film
#8
13-08-2012, 11:55 AM
 theodog (Jeff) Every photon is sacred ! Join Date: Oct 2007 Location: Coonabarabran Posts: 1,037
Quote:
 Originally Posted by Merlin66 I'm sure there are younger members who haven't even seen an ol' 35mm SLR.
Or Format

Has anybody used one of these.
http://www.cameracity.com.au/pentax-...a55mm-kit.html
#9
13-08-2012, 04:45 PM
 gregbradley Registered User Join Date: Feb 2006 Location: Sydney Posts: 15,516
Quote:
 Originally Posted by theodog Or Format Has anybody used one of these. http://www.cameracity.com.au/pentax-...a55mm-kit.html
From comparison reviews I have read between Nikon D800E and various Medium Format cameras, the medium format cameras seem to only work in a narrow ISO band like 100 - 400 and after that very poorly.

Most astro work is ISO800-6400 so medium format may be very disappointing for that. They seem to excel in studio controlled lighting conditions.

Greg.
#10
13-08-2012, 05:32 PM
 Steffen Ebotec Alpeht Sicamb Join Date: Feb 2010 Location: Toongabbie, NSW Posts: 1,884
Quote:
 Originally Posted by troypiggo It's not necessarily 35mm film, it's full frame sensor. Just so happens that full frame sensors are similar size to 35mm film
And of course users of medium format or view cameras would respectfully disagree with the term "full frame"

The problem with camera formats (sensor sizes) is that there are so many of them. This makes it hard for people to gauge what sort of lens they get when it has 6-24mm or 40-150mm printed on it. The (incorrectly termed) equivalent focal length allows people to distinguish between ultra-wide, wide, normal, short and long tele lenses by relating a 35mm format focal length with the same angle of view.

Alternatively, the angle of view could be specified directly but even fewer people would be familiar with that.

I expect the 35mm format reference to wane away in a couple of generations.

Cheers
Steffen.
#11
14-08-2012, 05:46 PM
 theodog (Jeff) Every photon is sacred ! Join Date: Oct 2007 Location: Coonabarabran Posts: 1,037
Quote:
 Originally Posted by gregbradley .......... to only work in a narrow ISO band like 100 - 400 and after that very poorly. Most astro work is ISO800-6400 so medium format may be very disappointing for that. They seem to excel in studio controlled lighting conditions. Greg.
This then begs the question, is ISO real or a ghost comparison to film standards? ie How is it measured now and compared to the "old" film standards?
#12
14-08-2012, 10:47 PM
 Garbz (Chris) Registered User Join Date: May 2012 Location: Brisbane Posts: 639
As someone who plays with both film and digital photography the ISO sensitivity of digital cameras is almost spot on with that of film.

For astrophotography however digital and film aren't even close. Digital is quite linear for lightness changes with exposure. Film however suffers from reciprocity failure a response which is almost logarithmic.

Suppose you take a 1 second exposure on ISO100 film. To increase the brightness one stop you need to go not to 2 seconds as expected, but rather to almost 4 seconds. 1 minute becomes 8 minutes, 4 minutes becomes 40 minutes, 10 minutes becomes 2 hours etc. So at these speeds film and digital are not comparable.

That said even at faster speeds I believe there's some 4 or so different ways to measure "ISO" on digital cameras depending on which standard you follow, just like there were several different ways for film.
#13
15-08-2012, 08:03 AM
 gregbradley Registered User Join Date: Feb 2006 Location: Sydney Posts: 15,516
I think also ISO settings are not necessarily identical between brands either.

Canon 5D3 and D800 video shows the Nikon ISO as much brighter for the same ISO number on the Canon for some reason. I am not sure if that applies to stills as well, perhaps only video.

It may be the higher QE of the Nikon is part of the reason.

Greg.

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