F2.8 is f4.5 on a cropped sensor

Started Apr 3, 2016 | Discussions
gezzamondo
gezzamondo Forum Member • Posts: 76
F2.8 is f4.5 on a cropped sensor

Is it true that if u buy a  f2.8 lens and use it on a cropped sensor body it will only be a maximum of f4.5

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johnierebel Senior Member • Posts: 2,100
Re: F2.8 is f4.5 on a cropped sensor

gezzamondo wrote:

Is it true that if u buy a f2.8 lens and use it on a cropped sensor body it will only be a maximum of f4.5

No, it is not. F2.8 is the same crop vs. full frame, the DOF will not be quite the same, though.

thomascantwell Forum Member • Posts: 53
Re: F2.8 is f4.5 on a cropped sensor

It's not quite that simple. A 50mm f/2.8 lens would have the field of view and depth of field of an 80mm f/4.5 lens on a full frame sensor.

gezzamondo
OP gezzamondo Forum Member • Posts: 76
Re: F2.8 is f4.5 on a cropped sensor

This guy's video on YouTube seems to think it will be

https://youtu.be/YDbUIfB5YUc

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johnierebel Senior Member • Posts: 2,100
Re: F2.8 is f4.5 on a cropped sensor

gezzamondo wrote:

This guy's video on YouTube seems to think it will be

https://youtu.be/YDbUIfB5YUc

Apple & oranges.  Aperture is how wide the opening is ( how much light is passing thru it relative to the focal length) . This article is about how the sensor captures the exposure. A full frame will have a thinner DOF. The F2.8 will allow faster shutter speeds than a F4.0, so you can stop motion blur from your subject at the cost/benefit of the shallower DOF.

J A C S
J A C S Forum Pro • Posts: 14,421
Re: F2.8 is f4.5 on a cropped sensor
richro Regular Member • Posts: 230
Re: F2.8 is f4.5 on a cropped sensor
5

If you're strictly talking about "what does the resulting image look like?" -- then yes, you would multiply both the focal length and aperture by the crop factor (1.6x for Canon APS-C) to get the 35mm equivalent (i.e. FF) field of view and depth of field, respectively. If you're not sure what the bolded terms mean, I think the simplest way is to think about the amount of "zoom" and "bokeh", respectively. As an example, a photo taken at 50mm f/2.8 on a Rebel T6i (APS-C) would look roughly similar to a photo taken at 80mm f/4.5 on a 6D (FF). (Note I am only talking about the FOV and DOF, not actual IQ/noise differences. Obviously, FF would have the advantage there.)

In pure technical terms, however, both the focal length and aperture of the lens do not change whether it's on a FF or crop body.

Hope this clears up any confusion!

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SarahBK
SarahBK Regular Member • Posts: 183
Re: F2.8 is f4.5 on a cropped sensor

It sort of does. It's mostly with regards to the depth of field being larger with the lens mounted on a crop sensor... but this is only if you compare another equivalent lens with the same actual aperture on a full frame camera (eg 50mm f/1.8 on a crop sensor and 85mm f/1.8 on a full frame camera). Mounting the same actual lens on a full frame camera changes up things even more - because a crop sensor camera makes any lens attached to it appear more 'zoomed' when compared to a full frame one. Which is a bit confusing and hence brings me to my second point...

All this only matters if you're considering a full frame camera. If you've never experienced a full frame camera and don't intend on buying or using one, the depth of field of an f/2.8 on a crop is just as you experience it. You'll know no different. If it's light gathering capability you're worried about, as far as I'm aware does not change depending on the size of the sensor.

Just be aware that with a full frame camera (or, even wider aperture lenses) will allow one to achieve shallower depths of field, either with more ease or simply making it possible. For example, you can never replicate the shallow depth of field and look of a 35mm f/1.4 + full frame camera on a crop sensor... because a wider aperture Canon 24mm does not exist (i.e. to create an equivalent lens for crop sensor cameras), and the 35mm mounted on a crop sensor has its effective focal length lengthened! In the latter case, because it is effectively more zoomed in, you need to take a few steps back to get the same composition of a 35mm+full frame and hence that effectively increases the depth of field.

What you could roughly replicate however would be, for example, the look of a 35mm f/2 on a full frame camera by using a 24mm f/1.4 lens on a crop sensor. The benefit of the latter would be that with that f/1.4 aperture you're still gathering way more light than the 35mm f/2. The downside? Just have a look at the price of the 24mm f/1.4 II...

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Jonathan Brady
Jonathan Brady Veteran Member • Posts: 6,725
Re: F2.8 is f4.5 on a cropped sensor

SarahBK wrote:

What you could roughly replicate however would be, for example, the look of a 35mm f/2 on a full frame camera by using a 24mm f/1.4 lens on a crop sensor. The benefit of the latter would be that with that f/1.4 aperture you're still gathering way more light than the 35mm f/2. The downside? Just have a look at the price of the 24mm f/1.4 II...

However the additional light gathered is cancelled out by the improved noise performance of FF. To determine equivalent ISO performance (assuming same tech in the sensors), you have to multiply the ISO by the crop factor, squared. So, in the case of Canon sensors, APSC is a 1.6 crop factor, so 1.6*1.6 = 2.56 or basically 2.5. So ISO 100 on crop is like ISO 250 on FF in terms of noise performance.

gezzamondo
OP gezzamondo Forum Member • Posts: 76
Re: F2.8 is f4.5 on a cropped sensor

My issue is that I currently have a cropped sensor on my Canon 760D and I want to invest in the 24-70mm f2.8 or even a 70-200 f2.8, basically to use for live music events where the light is low and the wide aperture would be great.

But, if I then buy these lenses only to find out out I can only get a maximum aperture of about f4.5 buy using my cropped sensor body id be extremely frustrated and feel short changed as these lenses aren't cheap and would defeat the purpose of buying them in the first place

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richro Regular Member • Posts: 230
Re: F2.8 is f4.5 on a cropped sensor

gezzamondo wrote:

My issue is that I currently have a cropped sensor on my Canon 760D and I want to invest in the 24-70mm f2.8 or even a 70-200 f2.8, basically to use for live music events where the light is low and the wide aperture would be great.

But, if I then buy these lenses only to find out out I can only get a maximum aperture of about f4.5 buy using my cropped sensor body id be extremely frustrated and feel short changed as these lenses aren't cheap and would defeat the purpose of buying them in the first place

It doesn't really "defeat the purpose" as you'll still be getting better low light images and bokeh at f/2.8 than the kit zoom lens, for example.

If you want even better performance, then you'll either have to spend the money to get a FF body -- or alternatively, get something like the Sigma 18-35 f/1.8 and Sigma 50-100 f/1.8. Personally, I made the upgrade to a 6D during a Canon refurb sale and think this is a better investment than spending big bucks on crop-only fast glass.

If you have a limited budget, your best bet right now is to pick up some cheap fast primes like the 50 f/1.8 STM and 24 f/2.8 STM for about $100 each. The 85 f/1.8 is about $350 and a great lens as well.

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Lemming51
Lemming51 Forum Pro • Posts: 15,037
no worries

gezzamondo wrote:

My issue is that I currently have a cropped sensor on my Canon 760D and I want to invest in the 24-70mm f2.8 or even a 70-200 f2.8, basically to use for live music events where the light is low and the wide aperture would be great.

Yes it would, though I'd argue for the EF-S 17-55 f/2.8 IS USM over the EF 24-70 f/2.8L II USM.  The EF-S has a more appropriate zoom range for a crop body user, has the same f/2.8 max aperture, is virtually as good optically and has the added benefit of image stabilization.

But, if I then buy these lenses only to find out out I can only get a maximum aperture of about f4.5 buy using my cropped sensor body id be extremely frustrated and feel short changed as these lenses aren't cheap and would defeat the purpose of buying them in the first place

No worries.  You will get the maximum aperture of f/2.8 on your cropped sensor.  24 mm and f/2.8 setting with the EF-S 17-55 f/2.8 IS USM on your 760D is exactly the same as 24mm and f/2.8 setting with the EF 24-70 f/2.8L II USM on your 760D.

And leaving aside the sensor performance difference of the 760D and 6D, the 24-70 f/2.8L II USM at any specific --mm will give exactly the same image projected onto your 760D sensor as it does on a 6D.  The "effective" differences result from (a) the smaller sensor effectively cropping the center of that image (thus effective focal length of 1.6x), and (b) the greater DOF on the crop is primarily the result of using a shorter focal length than you would on the full frame camera to get the same perspective and framing.  e.g. on a 760D 24mm @ f/2.8 gives the same perspective, framing, and depth of field as a 38mm @ f/4.5 on a 6D.

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Jonathan Brady
Jonathan Brady Veteran Member • Posts: 6,725
Yup!

It sounds like you're worried you won't be able to set the aperture to f/2.8, and that's not the case. You can still set it to f/2.8, and you'll get more light on the sensor than any lenses with a smaller max aperture than f/2.8.

I also agree with the recommendation to get the EF-S 17-55 f/2.8 IS lens instead of the 24-70 f/2.8 II. UNLESS... You plan to buy a full frame camera soon, which, if you're going to buy the 24-70 II and 70-200 f/2.8 IS II, you might as well go ahead and grab a 6D used, refurbished, or on one of the great deals new (assuming you're in the US).

trulandphoto
trulandphoto Contributing Member • Posts: 874
Re: F2.8 is f4.5 on a cropped sensor

richro wrote:

If you're strictly talking about "what does the resulting image look like?" -- then yes, you would multiply both the focal length and aperture by the crop factor (1.6x for Canon APS-C) to get the 35mm equivalent (i.e. FF) field of view and depth of field, respectively. If you're not sure what the bolded terms mean, I think the simplest way is to think about the amount of "zoom" and "bokeh", respectively. As an example, a photo taken at 50mm f/2.8 on a Rebel T6i (APS-C) would look roughly similar to a photo taken at 80mm f/4.5 on a 6D (FF). (Note I am only talking about the FOV and DOF, not actual IQ/noise differences. Obviously, FF would have the advantage there.)

In pure technical terms, however, both the focal length and aperture of the lens do not change whether it's on a FF or crop body.

Hope this clears up any confusion!

I also think that depth of field difference only comes into play if you move around to frame the scene the same with each sensor. If you don't move, keep the same settings, and just switch sensor sizes the image from the crop sensor will have the same DOF as that part of the FF image.

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Ken Croft Senior Member • Posts: 1,697
Re: no worries
2

What a long winded response to a very simple question.

In answer to the original question, as far as exposure is concerned your f2.8 lens is an f2.8 lens what ever body you put it on. Don't worry about it. The whole "equivalence" confusion has been done to death. Just get on taking photos.

Ken C

Erik Magnuson Forum Pro • Posts: 12,247
The type of question "equivalence" would answer
1

gezzamondo wrote:

My issue is that I currently have a cropped sensor on my Canon 760D and I want to invest in the 24-70mm f2.8 or even a 70-200 f2.8, basically to use for live music events where the light is low and the wide aperture would be great.

is more like: if I have $nnnn to spend and I want to shoot in low light (or with shallow DOF), am I better off spending it on a larger sensor or faster lenses for the smaller sensor?  E.g.  if it would cost you the same to buy the Sigma 50-100mm f/1.8 for your crop body or a (used) 6D and Sigma 70-200mm f/2.8, which would give better results?  (The question is harder because there are almost no exact matches in cost vs capability vs. portability)

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Erik

jitteringjr Veteran Member • Posts: 3,608
Re: F2.8 is f4.5 on a cropped sensor

A 70-200/2.8 is going to let in four times more light than your 55-250 at the long end.  That is the difference of ISO 6400 and ISO 1600.

Now say you are shooting crop and need 200mm and 2.8 but you still want better images.  That is where things get kind of stupid expensive for a hobbyist.  If you go FF then you loose the reach advantage.  So that 200/2.8 lens on full frame is giving you the reach that a 125mm lens on crop would give.  So you need to buy something like a 300/4 IS or a 100-400 to gain back the reach.  Problem is then you loose the aperture advantage and you are back to where you started.

So to gain an advantage, you need to spend $3000 on a Sigma 120-300 or used car prices on a Canon 300/2.8.

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Holscen Contributing Member • Posts: 564
Buy the f2.8
3

A full frame lenses of f2.8 will have the same aperture light gathering ability on a crop body as they do on full frame

however you will have to physically move back with a cropped sensor to get the same sized foreground subject, or performer, framed in the same way.

As you are now at a greater subject distance your depth of field and angle of view as well as the appearance of background compression in relation to your subject will change .

Roughly the dof will be multiplied by the 1.6 crop factor to just over a stop. So f2.8 will have the visual effect of f4.5 but the light gathering of 2.8.

Make the high density pixels of the crop sensor work for you by capturing all the smaller detail of ultra wides. A telephoto lens like a 200 may give the impression of a 320 on a crop, but it's just that the edges of the image ( that would be seen in full frame) fall outside the smaller crop sensor giving the illusion that your reach has been extended.

I would always buy full frame lenses, 2.8 or faster so you can use them on both formats.

I have shot 1000s of gigs with both crop & full frame formats and I guarantee the crop body was not a stop darker, it just had that's a very different look

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Phil Geusebroek Contributing Member • Posts: 622
Re: F2.8 is f4.5 on a cropped sensor

gezzamondo wrote:

Is it true that if u buy a f2.8 lens and use it on a cropped sensor body it will only be a maximum of f4.5

To my understanding, using a 24mm f/2.8 on EOS crop as an example:

- Crop field of view is about 38.5mm effective focal length, but compression is same as 24mm FF

- Light on the sensor is the same as f/4.5 on a FF sensor.

- DOF is same as f/2.8 on FF.

Great Bustard Forum Pro • Posts: 42,945
With regards to *both* DOF and noise, yes.
2

gezzamondo wrote:

Is it true that if u buy a f2.8 lens and use it on a cropped sensor body it will only be a maximum of f4.5

More properly, one should say that f/2.8 on 1.6x is equivalent to (as opposed to "equal to) f/4.5 on FF just as 50mm on 1.6x is equivalent to (as opposed to "equal to") 80mm on FF.

In any case, we say that 50mm on 1.6x is equivalent to 80mm on FF because they result in the same angle of view.  Likewise, f/2.8 on 1.6x results in the same aperture diameter as f/4.5 on FF, which in turn results in the same DOF and same total amount of light on the sensor (if the exposure times are the same), which in turn results in the same noise (assuming sensors that record the same proportion of light that falls on them and add in the same amount of electronic noise).

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