upcoming Oly 40-150 f2.8 or ....

How low actually is the light you're going to be working in? You need to distinguish between the general lighting level in the theatre and the lighting level on the actual performers which is going to be considerably higher than the general lighting level.

Try using the spot meter and measuring a small area of the performers the next time you shoot a performance. You're going to have to try and pick a mid tone or you can choose a caucasian skin area and give an extra stop exposure compensation over what the meter recommends. If you've been using the standard whole sensor averaging meter mode and you're including a fair amount of dark background in the frame around the performers I think you'll find that you won't need to boost your ISO setting as much with the spot meter reading. The same is likely if you're using centre weighted though the difference between the centre weighted reading and the spot meter reading is likely to be less than the difference between the normal averaging mode and the spot meter reading.

I've used the 75mm to shoot birds in shadowed areas because it was faster than the current 40-150mm zoom. It let me work at much lower ISO settings than I need with the 40-150 and it covers only 4 times the area of the 40-150 at 150mm. I've got good results cropping to an area smaller than the zoom would give at 150mm but you couldn't print/show them at as large a size as you could a full frame at 150mm allows you to do. Depending on how large your images are going to be displayed, and how close you can get, you may very well be very happy with the 75mm lens for the sort of work you're talking about. It's a very sharp lens with very nice rendering.

If you use the current 40-150 at 150mm your maximum aperture is F/6.3 which is almost 3 stops slower than the 75mm. That's a really big difference in low light and it's basically the difference between ISO 200 and ISO 160, or between ISO 800 and ISO 3200. You may well not want to use the current 40-150 at anything longer than 75mm anyway if you're concerned about raising your ISO level so you're first choice might even be able to be simplified to the 75mm on its own, rather than the 75 and the current 40-150.

The new 40-150 PRO zoom is certainly going to be faster than the current one but if you can get what you want with the 25 and the 75, that's an even faster option and the 75 costs less than the new zoom will.

My advice is borrow or hire a 75 and try it. Alternatively take a look at how much you're cropping your images from the 25mm. If you look at a shot from the 25, what the 75 would have given you from the same shooting spot is a frame one third of the width and one third of the height of what the 25 delivered. If you never need to crop to a smaller area than that, then the 75mm is as long as you need. The only thing it won't deliver that a 40-150 would is the ability to shoot full frames at focal lengths between 40 and 75mm, You'd need to crop images from the 25 to get those fields of view but the 25 will be faster than either zoom in that range anyway and that will keep your ISO setting down which will help.

If raising ISO is a real concern the 75 is certainly the better way to go and if it gets you close enough so that you rarely want anything longer than 75 then why consider either zoom at all? I think it all boils down to whether or not 75 is long enough to let you get what you want, perhaps with modest cropping, or whether you really need all of the reach that you would get at the long end of the range of the 40-150.

 

I really don't think the 75mm will perform 'much' better, as you're only talking about slightly more than a one stop advantage, if you're talking about the 40-150mm f2.8.

 

I shoot plays and dimly lit events quite often with my m43 gear. I used to shoot with two primes (pl 25mm f1.4 + Rokinon 85mm f1.4) with an epm2 and a gx7. I've since upgraded my wide angle body to be fitted with a 12-40mm f2.8. I shot my last show using a combination of this lens along with the 85mm f1.4.

In a fairly well lit (for a play) production, at f2.8, I was shooting around ISO 1000-3200 with a shutter speed of around 1/60-1/80, depending on the scene. My 25mm f1.4 would usually hover around ISO 400-800 for the same type of lighting situations. f2.8 really is a bit narrow of an aperture for theater performance type stuff, but can be compensated for by the slower shutter speed you can get away with at the short end of the focal range. The shots that the 12-40 gave me were really high quality and I didn't mind bumping the ISO up, but I think 3200 is pushing it a bit. Despite that, my favorite shots were with the Rokinon as I found this to be just the right focal length for many of the close up shots I wanted. I was able to keep a high shutter speed while keeping the ISO within a more useable range. f1.4-2 also provided really nice subject isolation and bokeh.

I'm sure the 40-150 f2.8 would handle the conditions you shoot in just fine as long as you are willing to go as high as 3200. One thing that you have to consider is that due to the longer focal length, especially at the long end, you'd have to shoot at a pretty high shutter speed to make sure that the shot is steady. 3 axis stabilization or not, I'm sure it would be difficult to get non blurry images in low light at shutter speeds even around 1/60 or so. My feeling is that if 75mm ends up being a workable focal length for you, go with that one. I just got one and it really is a special lens. I've found that shooting two bodies is the best way to go in low light conditions when you have to change focal lengths. As an alternate solution, you could rent a Panasonic 35-100 f2.8 for an event and see how that suits you. If all your shots were around ISO 3200 or even higher, you may have to go with the faster prime.

All that being said, the 40-150 f4-5.6 is also an amazing lens. In well lit conditions, like the field days or swim meets you mention, it produces some amazingly sharp images. At its present price, it's a rather low cost investment and will satisfy your needs for the outdoor stuff.

 

Use spot metering w/ exposure compensation as noted above.

The 40-150 f4/5.6 won't be a good choice.

Try the 75, 45 or 35-100 f/2.8. Or a bright legacy MF zoom/prime if you are okay with MF.

 

If you're considering renting, there are also the fast four-thirds zooms, which you can adapt -

http://www.four-thirds.org/en/fourthirds/telephoto.html

May be a little unwieldy if you don't have support. I know 43's AF on the EM1 improved markedly but I'm unsure if that performance passed through to the EM10.

 

I wouldn't be putting any good 4/3's glass on the E-M10 (unless it is the 14-54 II lens) because of the poor AF (but would go well on an E-M1), but the 40-150 f/2.8 would allow for most stage type photos with regards to ISO/shutter speed, with the ability to zoom in for those special moments of an individual & zoom out for that particular group shot. Ideally two bodies are better, but not everyone can afford that luxury.