Crop factor for aperture

omens

omens

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I thought I'd gotten my head around the issue of lenses and FF/APSC.

But then I watched this:

Now I love Tony northrup's approach with his photography. But this has now given me more of a headache. Do I really need to multiply aperture by 1.6x to get the true aperture on my 70D?

Does that mean I need an f/1.8 to get f/2.8 on my 70D?

I'm starting to wish i'd gone full frame to avoid the headaches.
 
omens said:
I thought I'd gotten my head around the issue of lenses and FF/APSC.

But then I watched this:

Now I love Tony northrup's approach with his photography. But this has now given me more of a headache. Do I really need to multiply aperture by 1.6x to get the true aperture on my 70D?

Does that mean I need an f/1.8 to get f/2.8 on my 70D?

I'm starting to wish i'd gone full frame to avoid the headaches.
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Well...
RichardTaylor said:
No .... aperture is a function of the lens design, and possibly any lens attachments like extension tubes.
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Indeed it is!
But to get the equivalent DoF, you need a wider aperture, I never watched the video, it's an educated guess, based on my casual interest in photography and a basic understanding of physics.

However, for the OP, there is no headache, stop trying to measure your crop camera by measuring it against FF.

Full Frame isn't a standard. There have always been different film sizes, we just got on with using what we had, so a standard lens on 135 might have been 50mm, but on square medium format it was 80mm, on larger cameras it was bigger still, on disc cameras it was tiny. No-one ever suggested a crop factor for focal length or for aperture (although mathematically it was a reality).

With APSC digital, someone somewhere in marketing coined the phrase 'crop factor' and put a number on it. It's the source of acres of debate, because people misunderstand it. The reality is of course, it's not really a 'thing' we have small medium and large family cars, we don't measure them all against a mk1 Golf! A Mondeo would be a 1.5 and a New Fiesta would be a 1, but an older fiesta would be a .8. It's flipping madness, there has never been a standard film/ sensor size in almost 200 years of image making. Why start comparing to 'full frame' now?
 
omens said:
Now I love Tony northrup's approach with his photography. But this has now given me more of a headache. Do I really need to multiply aperture by 1.6x to get the true aperture on my 70D?

Does that mean I need an f/1.8 to get f/2.8 on my 70D?

I'm starting to wish i'd gone full frame to avoid the headaches.
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....What I love about Tony Northrup is Chelsea Northrup!

You don't need to do any calculations - Simply shoot what you see in the viewfinder. Shoot, shoot, shoot, and check your aperture and other settings on those photos you took which you like the best AFTERWARDS. Thereby you learn how each lens you use performs and which settings achieve which results.

Equivalent DoF (Depth of Focus Field) between sensor sizes and formats is academic and doesn't help your photography.
 
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@omens I feel your pain. I too confused myself about things like is a 35mm on a crop camera equal to a 50mm on a FF.
In the end, as the other posters have said, it makes no difference unless you are using for motoring enforcement or the like.
Just let it go and don't worry about it :)
 
Just a few more thoughts, from a hobbyst who shoots with small sensor (P&S), crop sensor (DSLR) and full frame (24x36mm DSLR) sensor cameras.

Most of the time my lens selection will be determined by:
(a) What do I need to fill the frame with the subject
(b) what will the lighting conditions be like.

My aperture selection will mostly be determined by
(a) what are the lighting conditions like (sometimes you may need to make compromises in very low light with ISO selection)?
(b) how much dof do I want?
 
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I figured the change in aperture would remain the same so if I shot using an F/4 lens and decided I wanted a faster lens, I'd still get benefit with F/2.8 over F/4 although it wouldn't behave as a 2.8 on a crop.
 
omens said:
I figured the change in aperture would remain the same so if I shot using an F/4 lens and decided I wanted a faster lens, I'd still get benefit with F/2.8 over F/4 although it wouldn't behave as a 2.8 on a crop.
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You're overthinking it!

As far as light gathering is concerned, the aperture is a constant. For DoF you have to live with the fact that DoF is greater on smaller sensors (or films) and shallower on larger.
 
Phil V said:
You're overthinking it!

As far as light gathering is concerned, the aperture is a constant. For DoF you have to live with the fact that DoF is greater on smaller sensors (or films) and shallower on larger.
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Indeed.

I always work on the theory that if you multiple aperture by crop factor you're not going to be far wrong on DoF. So an f/2 setting on a 1.6 crop camera will give the same DoF as f/3.2 using the same lens on full frame
 
Northrup should not have made that video. Northrup appears to makes the mistake of using full frame as a standard. An unbelievable mistake for someone as conversant as him with photography. If you apply the rules he has made to medium format and 10x8 plate cameras that would mean all the full frame f stop figures are wrong. As regards the depth of field and sensor size, it is not so much the sensor size rather it is it's distance from the back of the lens.The closer any size sensor gets to the back of the lens the narrower the beams of light striking it are so therefore the more compact depth of field. Mr Northrup appears to be having a David Icke moment. Mr Northrup men did really land on the moon. I hope you recover soon. :beer:
 
Personally I think people make too much of this.

If exposure is the topic it's not an issue and f2.8 gives the same aperture on whatever...

If DoF is the topic you can take the crop factor into account and then there are +'s and -'s. One + is, for example, that you can get FF f2.8 DoF at f1.4 on MFT (MFT just to keep the numbers easy as it's x2 crop) which means you can use a lower ISO or a faster shutter speed. Another + is that with a smaller system you can get more DoF at wider apertures and get both of someones eyes in the DoF. The - is that if you are going for wafer thin DoF you have to use very wide apertures if you're using a smaller format, but... how often do we really need wafer thin DoF?

My experience has been that often with a FF camera I'm trying to get more DoF not less and once you stop down to f2.8 you may well be able to get the same DoF from a smaller system relatively easily. Plus if you're going for wafer thin DoF when using a smaller system you can get it if you can reduce your camera to subject distance, it'll be a different framing but that may not be the end of the wold.

PS. I think that if you grew up with 35mm or something bigger and change to a smaller format it's worth remembering the crop factor so that you can alter your settings appropriately. For example if shooting with a FF 5D and 50mm I may think that I want to be at f8 but when shooting with my MFT camera and 25mm lens I can get the same image, DoF wise, at f4 and there's maybe little point stopping down to f8.
 
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This is all to do with the confusion between aperture and focal ratio. They are closely related, but definitely not the same. Most photographers use the word aperture where focal ratio should infact be used.
At the end of the day, f4 is f4. That doesnt change no matter what the size of your sensor. The same aperture at the same focal ratio will give the same image brightness, even if light is being "wasted" either side of your sensor.
Thank god you dont have to factor in eyepiece focal lengths and exit pupils :P
 
The video does have a point where lenses designed for crop factor sensors are marketed as 100mm eq=150mm f2.8 as it is eq to 150mm 4.2ish.
I want a 26-134mm f1.4 NOW :)
 
Peter69 said:
The video does have a point where lenses designed for crop factor sensors are marketed as 100mm eq=150mm f2.8 as it is eq to 150mm 4.2ish.
I want a 26-134mm f1.4 NOW :)
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Can you explain what you are saying?Are you saying that a stop of light goes missing somewhere?
 
No, that would be iso.
Use a dof calculator online to see the dof differance between sensor sizes.
 
Peter69 said:
The video does have a point where lenses designed for crop factor sensors are marketed as 100mm eq=150mm f2.8 as it is eq to 150mm 4.2ish.
I want a 26-134mm f1.4 NOW :)
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No it's not!
The supposition is bad, the maths is terrible :)

1 stop more than 2.8 is 4. So 4.2 would be equivalent to a greater than 2x crop factor. 3.6 would be close (ish). Add in the other important issue shooting Macro, which is workable focus distance, and that's based on actual focal length and has no regard for crop factors.

All of which proves my point that it makes more sense to know how your gear behaves rather than comparing it to other gear. Particularly as most people don't have more than one format, so they're making comparisons against theoretical kit.
 
What on earth has Northrup started.
It's like the story about the man who gave £10 to 3 of his workers ( he did not trust one of them with all the money) to go and pick up a car that was for sale at £30. When they got there it was only £25. So they put £2 in petrol in the tank and kept a £1 each. When they got back they told the boss that the car cost £25 and they each gave him the £1 back and told him that they had put £2 of petrol in the tank. But the boss used Northrup maths and said "hang on I gave you each £10 and you have each given me a £1 back so that means you paid £9 each for the car that's £27 and put £2 of petrol in the tank so that's £29 where is the other £1". "They said ask Tony Northrup only he knows". :runaway:
 
@Phil V, I was being a little sarcastic.
I know the maths. I have a DX sensor and don't worry about 'equivalent' focal lengths, apertures or ISO.

As some once said "ye canna change the laws of physics cap'n"
 
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