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

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

https://youtu.be/YDbUIfB5YUc

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!

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...

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

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.

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.

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.

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.

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